Methods for driving bistable electro-optic displays, and apparatus for use therein

ABSTRACT

A bistable electro-optic display has a plurality of pixels, each of which is capable of displaying at least three gray levels. The display is driven by a method comprising: storing a look-up table containing data representing the impulses necessary to convert an initial gray level to a final gray level; storing data representing at least an initial state of each pixel of the display; receiving an input signal representing a desired final state of at least one pixel of the display; and generating an output signal representing the impulse necessary to convert the initial state of said one pixel to the desired final state thereof, as determined from said look-up table. The invention also provides a method for reducing the remnant voltage of an electro-optic display.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from the following ProvisionalApplications: (a) Serial No. 60/319,007, filed Nov. 20, 2001; (b) SerialNo. 60/319,010, filed Nov. 21, 2001; (c) Serial No. 60/319,034, filedDec. 18, 2001; (d) Serial No. 60/319,037, filed Dec. 20, 2001; and (e)Serial No. 60/319,040, filed Dec. 21, 2001. This application is also acontinuation-in-part of copending application Ser. No. 09/561,424, filedApr. 28, 2000, which is itself a continuation-in-part of copendingapplication Ser. No. 09/520,743, filed Mar. 8, 2000. The entire contentsof the aforementioned applications are herein incorporated by reference.

BACKGROUND OF INVENTION

[0002] This invention relates to methods for driving bistableelectro-optic displays, and to apparatus for use in such methods. Morespecifically, this invention relates to driving methods and apparatuscontroller which are intended to enable more accurate control of graystates of the pixels of an electro-optic display. This invention alsorelates to a method which enables long-term direct current (DC)balancing of the driving impulses applied to an electrophoretic display.This invention is especially, but not exclusively, intended for use withparticle-based electrophoretic displays in which one or more types ofelectrically charged particles are suspended in a liquid and are movedthrough the liquid under the influence of an electric field to changethe appearance of the display.

[0003] In one aspect, this invention relates to apparatus which enableselectro-optic media which are sensitive to the polarity of the appliedfield to be driven using circuitry intended for driving liquid crystaldisplays, in which the liquid crystal material is not sensitive topolarity.

[0004] The term “electro-optic” as applied to a material or a display,is used herein in its conventional meaning in the imaging art to referto a material having first and second display states differing in atleast one optical property, the material being changed from its first toits second display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

[0005] The term “gray state” is used herein in its conventional meaningin the imaging art to refer to a state intermediate two extreme opticalstates of a pixel, and does not necessarily imply a black-whitetransition between these two extreme states. For example, several of thepatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned the transition between the two extremestates may not be a color change at all.

[0006] The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin copending application Ser. No. 10/063,236, filed Apr. 2, 2002 (seealso the corresponding International Application Publication No. WO02/079869) that some particle-based electrophoretic displays capable ofgray scale are stable not only in their extreme black and white statesbut also in their intermediate gray states, and the same is true of someother types of electro-optic displays. This type of display is properlycalled “multi-stable” rather than bistable, although for convenience theterm “bistable” may be used herein to cover both bistable andmulti-stable displays.

[0007] The term “gamma voltage” is used herein to refer to externalvoltage references used by drivers to determine voltages to be appliedto pixels of a display. It will be appreciated that a bistableelectro-optic medium does not display the type of one-to-one correlationbetween applied voltage and optical state characteristic of liquidcrystals, the use of the term “gamma voltage” herein is not preciselythe same as with conventional liquid crystal displays, in which gammavoltages determine inflection points in the voltage level/output voltagecurve.

[0008] The term “impulse” is used herein in its conventional meaning ofthe integral of voltage with respect to time. However, some bistableelectro-optic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

[0009] Several types of bistable electro-optic displays are known. Onetype of electro-optic display is a rotating bichromal member type asdescribed, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782;5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and6,147,791 (although this type of display is often referred to as a“rotating bichromal ball” display, the term “rotating bichromal member”is preferred as more accurate since in some of the patents mentionedabove the rotating members are not spherical). Such a display uses alarge number of small bodies (typically spherical or cylindrical) whichhave two or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface.

[0010] Another type of electro-optic medium uses an elecrochromicmedium, for example an electrochromic medium in the form of ananochromic film comprising an electrode formed at least in part from asemi-conducting metal oxide and a plurality of dye molecules capable ofreversible color change attached to the electrode; see, for exampleO'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., InformationDisplay, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater.,2002, 14(11), 845. Nanochromic films of this type are also described,for example, in U.S. Pat. No. 6,301,038, International ApplicationPublication No. WO 01/27690, and in copending Applications Serial Nos.60/365,368; 60/365,369; 60/365,385 and 60/365,365, all filed Mar. 18,2002, Applications Serial Nos. 60/319,279; 60/319,280; and 60/319,281,all filed May 31, 2002; and Application Serial No. 60/319,438, filedJul. 31, 2002.

[0011] Another type of electro-optic display, which has been the subjectof intense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

[0012] Numerous patents and applications assigned to or in the names ofthe Massachusetts Institute of Technology (MIT) and E Ink Corporationhave recently been published describing encapsulated electrophoreticmedia. Such encapsulated media comprise numerous small capsules, each ofwhich itself comprises an internal phase containingelectrophoretically-mobile particles suspended in a liquid suspensionmedium, and a capsule wall surrounding the internal phase. Typically,the capsules are themselves held within a polymeric binder to form acoherent layer positioned between two electrodes. Encapsulated media ofthis type are described, for example, in U.S. Pat. Nos. 5,930,026;5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839;6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950;6,249,721; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304;6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785;6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; and 6,459,418;and U.S. patent applications Publication Nos. 2001/0045934;2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661;2002/0063677; 2002/0090980; 2002/106847; 2002/0113770; 2002/0130832;2002/0131147; and 2002/0154382, and International ApplicationsPublication Nos. WO 99/53373; WO 99/59101; WO 99/67678; WO 00/05704; WO00/20922; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/20922; WO00/36666; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO01/17029; and WO 01/17041.

[0013] Many of the aforementioned patents and applications recognizethat the walls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, WO 01/02899, at page 10,lines 6-19. See also copending application Ser. No. 09/683,903, filedFeb. 28, 2002, and the corresponding International ApplicationPCT/US02/06393. Accordingly, for purposes of the present application,such polymer-dispersed electrophoretic media are regarded as sub-speciesof encapsulated electrophoretic media.

[0014] An encapsulated electrophoretic display typically does not sufferfrom the clustering and settling failure mode of traditionalelectrophoretic devices and provides further advantages, such as theability to print or coat the display on a wide variety of flexible andrigid substrates. (Use of the word “printing” is intended to include allforms of printing and coating, including, but without limitation:pre-metered coatings such as patch die coating, slot or extrusioncoating, slide or cascade coating, curtain coating; roll coating such asknife over roll coating, forward and reverse roll coating; gravurecoating; dip coating; spray coating; meniscus coating; spin coating;brush coating; air knife coating; silk screen printing processes;electrostatic printing processes; thermal printing processes; ink jetprinting processes; and other similar techniques.) Thus, the resultingdisplay can be flexible. Further, because the display medium can beprinted (using a variety of methods), the display itself can be madeinexpensively.

[0015] A related type of electrophoretic display is a so-called“microcell electrophoretic display”. In a microcell electrophoreticdisplay, the charged particles and the suspending fluid are notencapsulated within microcapsules but instead are retained within aplurality of cavities formed within a carrier medium, typically apolymeric film. See, for example, International Applications PublicationNo. WO 02/01281, and published U.S. application No. 2002-0075556, bothassigned to Sipix Imaging, Inc.

[0016] The bistable or multi-stable behavior of particle-basedelectrophoretic displays, and other electro-optic displays displayingsimilar behavior, is in marked contrast to that of conventional liquidcrystal (“LC”) displays. Twisted nematic liquid crystals act are not bi-or multi-stable but act as voltage transducers, so that applying a givenelectric field to a pixel of such a display produces a specific graylevel at the pixel, regardless of the gray level previously present atthe pixel. Furthermore, LC displays are only driven in one direction(from non-transmissive or “dark” to transmissive or “light”), thereverse transition from a lighter state to a darker one being effectedby reducing or eliminating the electric field. Finally, the gray levelof a pixel of an LC display is not sensitive to the polarity of theelectric field, only to its magnitude, and indeed for technical reasonscommercial LC displays usually reverse the polarity of the driving fieldat frequent intervals.

[0017] In contrast, bistable electro-optic displays act, to a firstapproximation, as impulse transducers, so that the final state of apixel depends not only upon the electric field applied and the time forwhich this field is applied, but also upon the state of the pixel priorto the application of the electric field. Furthermore, it has now beenfound, at least in the case of many particle-based electro-opticdisplays, that the impulses necessary to change a given pixel throughequal changes in gray level (as judged by eye or by standard opticalinstruments) are not necessarily constant, nor are they necessarilycommutative. For example, consider a display in which each pixel candisplay gray levels of 0 (white), 1, 2 or 3 (black), beneficially spacedapart. (The spacing between the levels may be linear in percentagereflectance, as measured by eye or by instruments but other spacings mayalso be used. For example, the spacings may be linear in L*, or may beselected to provide a specific gamma; a gamma of 2.2 is often adoptedfor monitors, and where the present displays are be used as areplacement for a monitor, use of a similar gamma may be desirable.) Ithas been found that the impulse necessary to change the pixel from level0 to level 1 (hereinafter for convenience referred to as a “0-1transition”) is often not the same as that required for a 1-2 or 2-3transition. Furthermore, the impulse needed for a 1-0 transition is notnecessarily the same as the reverse of a 0-1 transition. In addition,some systems appear to display a “memory” effect, such that the impulseneeded for (say) a 0-1 transition varies somewhat depending upon whethera particular pixel undergoes 0-0-1, 1-0-1 or 3-0-1 transitions. (Where,the notation “x-y-z”, where x, y, and z are all optical states 0, 1, 2,or 3 denotes a sequence of optical states visited sequentially in time.)Although these problems can be reduced or overcome by driving all pixelsof the display to one of the extreme states for a substantial periodbefore driving the required pixels to other states, the resultant“flash” of solid color is often unacceptable; for example, a reader ofan electronic book may desire the text of the book to scroll down thescreen, and may be distracted, or lose his place, if the display isrequired to flash solid black or white at frequent intervals.Furthermore, such flashing of the display increases its energyconsumption and may reduce the working lifetime of the display. Finally,it has been found that, at least in some cases, the impulse required fora particular transition is affected by the temperature and the totaloperating time of the display, and by the time that a specific pixel hasremained in a particular optical state prior to a given transition, andthat compensating for these factors is desirable to secure accurate grayscale rendition.

[0018] In one aspect, this invention seeks to provide a method and acontroller that can provide accurate gray levels in an electro-opticdisplay without the need to flash solid color on the display at frequentintervals.

[0019] Furthermore, as will readily be apparent from the foregoingdiscussion, the drive requirements of bistable electro-optic mediarender unmodified drivers designed for driving active matrix liquidcrystal displays (AMLCD's) unsuitable for use in bistable electro-opticmedia-based displays. However, such AMLCD drivers are readily availablecommercially, with large permissible voltage ranges and high pin-countpackages, on an off-the-shelf basis, and are inexpensive, so that suchAMLCD drives are attractive for drive bistable electro-optic displays,whereas similar drivers custom designed for bistable electro-opticmedia-based displays would be substantially more expensive, and wouldinvolve substantial design and production time. Accordingly, there arecost and development time advantages in modifying AMLCD drivers for usewith bistable electro-optic displays, and this invention seeks toprovide a method and modified driver which enables this to be done.

[0020] Also, as already noted, this invention relates to methods fordriving electrophoretic displays which enable long-term DC-balancing ofthe driving impulses applied to the display. It has been found thatencapsulated and other electrophoretic displays need to be driven withaccurately DC-balanced waveforms (i.e., the integral of current againsttime for any particular pixel of the display should be held to zero overan extended period of operation of the display) to preserve imagestability, maintain symmetrical switching characteristics, and providethe maximum useful working lifetime of the display. Conventional methodsfor maintaining precise DC-balance require precision-regulated powersupplies, precision voltage-modulated drivers for gray scale, andcrystal oscillators for timing, and the provision of these and similarcomponents adds greatly to the cost of the display.

[0021] Furthermore, even with the addition of such expensive components,true DC balance is still not obtained. Empirically it has been foundthat many electrophoretic media have asymmetric current/voltage (I/Vcurves); it is believed, although the invention is in no way limited bythis belief, that these asymmetric curves are due to electrochemicalvoltage sources within the media. These asymmetric curves mean that thecurrent when the medium is addressed to one extreme optical state (sayblack) is not the same as when the medium is addressed to the opposedextreme optical state (say white), even when the voltage is carefullycontrolled to be precisely the same in the two cases.

[0022] It has now been found that the extent of DC imbalance in anelectrophoretic medium used in a display can be ascertained by measuringthe open-circuit electrochemical potential (hereinafter for conveniencecalled the “remnant voltage” of the medium. When the remnant voltage ofa pixel is zero, it has been perfectly DC balanced. If its remnantvoltage is positive, it has been DC unbalanced in the positivedirection. If its remnant voltage is negative, it has been DC unbalancedin the negative direction. This invention uses remnant voltage data tomaintain long-term DC-balancing of the display.

SUMMARY OF INVENTION

[0023] Accordingly, in one aspect, this invention provides a method ofdriving a bistable electro-optic display having a plurality of pixels,each of which is capable of displaying at least three gray levels (as isconventional in the display art, the extreme black and white states areregarded as two gray levels for purposes of counting gray levels). Themethod comprises:

[0024] storing a look-up table containing data representing the impulsesnecessary to convert an initial gray level to a final gray level;

[0025] storing data representing at least an initial state of each pixelof the display;

[0026] receiving an input signal representing a desired final state ofat least one pixel of the display; and

[0027] generating an output signal representing the impulse necessary toconvert the initial state of said one pixel to the desired final statethereof, as determined from said look-up table.

[0028] This method may hereinafter for convenience be referred to as the“look-up table method” of the present invention.

[0029] This invention also provides a device controller for use in sucha method. The controller comprises:

[0030] storage means arranged to store both a look-up table containingdata representing the impulses necessary to convert an initial graylevel to a final gray level, and data representing at least an initialstate of each pixel of the display;

[0031] input means for receiving an input signal representing a desiredfinal state of at least one pixel of the display;

[0032] calculation means for determining, from the input signal, thestored data representing the initial state of said pixel, and thelook-up table, the impulse required to change the initial state of saidone pixel to the desired final state; and

[0033] output means for generating an output signal representative ofsaid impulse.

[0034] This invention also provides a method of driving a bistableelectro-optic display having a plurality of pixels, each of which iscapable of displaying at least three gray levels. The method comprises:

[0035] storing a look-up table containing data representing the impulsesnecessary to convert an initial gray level to a final gray level;

[0036] storing data representing at least an initial state of each pixelof the display;

[0037] receiving an input signal representing a desired final state ofat least one pixel of the display; and

[0038] generating an output signal representing the impulse necessary toconvert the initial state of said one pixel to the desired final statethereof, as determined from said look-up table, the output signalrepresenting the period of time for which a substantially constant drivevoltage is to be applied to said pixel.

[0039] This invention also provides a device controller for use in sucha method. The controller comprises:

[0040] storage means arranged to store both a look-up table containingdata representing the impulses necessary to convert an initial graylevel to a final gray level, and data representing at least an initialstate of each pixel of the display;

[0041] input means for receiving an input signal representing a desiredfinal state of at least one pixel of the display;

[0042] calculation means for determining, from the input signal, thestored data representing the initial state of said pixel, and thelook-up table, the impulse required to change the initial state of saidone pixel to the desired final state; and

[0043] output means for generating an output signal representative ofsaid impulse, the output signal representing the period of time forwhich a substantially constant drive voltage is to be applied to saidpixel.

[0044] In another aspect, this invention provides a device controllerfor use in the method of the present invention. The controllercomprises:

[0045] storage means arranged to store both a look-up table containingdata representing the impulses necessary to convert an initial graylevel to a final gray level, and data representing at least an initialstate of each pixel of the display;

[0046] input means for receiving an input signal representing a desiredfinal state of at least one pixel of the display;

[0047] calculation means for determining, from the input signal, thestored data representing the initial state of said pixel, and thelook-up table, the impulse required to change the initial state of saidone pixel to the desired final state; and

[0048] output means for generating an output signal representative ofsaid impulse, the output signal representing a plurality of pulsesvarying in at least one of voltage and duration, the output signalrepresenting a zero voltage after the expiration of a predeterminedperiod of time.

[0049] In another aspect, this invention provides a driver circuithaving output lines arranged to be connected to drive electrodes of anelectro-optic display. This driver circuit has first input means forreceiving a plurality of (n+1) bit numbers representing the voltage andpolarity of signals to be placed on the drive electrodes; and secondinput means for receiving a clock signal. Upon receipt of the clocksignal, the driver circuit displays the selected voltages on its outputlines. In one preferred form of this driver circuit, the selectedvoltages may be any one of 2^(n) discrete voltages between R and R+V,where R is a predetermined reference voltage (typically the voltage of acommon front electrode in an active matrix display, as described in moredetail below), and V is the maximum difference from the referencevoltage which the driver circuit can assert, or any one of 2^(n)discrete voltages between R and R−V. These selected voltages may belinearly distributed over the range of R±V, or may be distributed in anon-linear manner; the non-linearity may be controlled by two or moregamma voltages placed within the specified range, each gamma voltagedefining a linear regime between that gamma voltage and the adjacentgamma or reference voltage.

[0050] In another aspect, this invention provides a driver circuithaving output lines arranged to be connected to drive electrodes of anelectro-optic display. This driver circuit has first input means forreceiving a plurality of 2-bit numbers representing the voltage andpolarity of signals to be placed on the drive electrodes; and secondinput means for receiving a clock signal. Upon receipt of the clocksignal, the driver circuit displays the voltages selected from R+V, Rand R−V (where R and V are as defined above) on its output lines.

[0051] In another aspect, this invention provides a method for drivingan electro-optic display which displays a remnant voltage, especially anelectrophoretic display. This method comprises:

[0052] (a) applying a first driving pulse to a pixel of the display;

[0053] (b) measuring the remnant voltage of the pixel after the firstdriving pulse; and

[0054] (c) applying a second driving pulse to the pixel following themeasurement of the remnant voltage, the magnitude of the second drivingpulse being controlled dependent upon the measured remnant voltage toreduce the remnant voltage of the pixel.

[0055] This method may hereinafter for convenience be referred to as the“remnant voltage” method of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

[0056]FIG. 1 is a schematic representation of an apparatus of thepresent invention, a display which is being driven by the apparatus, andassociated apparatus, and is designed to show the overall architectureof the system;

[0057]FIG. 2 is a schematic block diagram of the controller unit shownin FIG. 1 and illustrates the output signals generated by this unit;

[0058]FIG. 3 is a schematic block diagram showing the manner in whichthe controller unit shown in FIGS. 1 and 2 generates certain outputsignals shown in FIG. 2;

[0059]FIGS. 4 and 5 illustrate two different sets of reference voltageswhich can be used in the display shown in FIG. 1;

[0060]FIG. 6 is a schematic representation of tradeoffs between pulsewidth modulation and voltage modulation approaches in the look-up tablemethod of the present invention;

[0061]FIG. 7 is a block diagram of a custom driver useful in the look-uptable method of the present invention;

[0062]FIG. 8 is a flow chart illustrating a program which may be run bythe controller unit shown in FIGS. 1 and 2;

[0063]FIGS. 9 and 10 illustrate two drive schemes of the presentinvention; and

[0064]FIGS. 11A and 11B illustrate two parts of a third drive scheme ofthe present invention.

DETAILED DESCRIPTION

[0065] As already mentioned, the look-up table aspect of the presentinvention provides methods and controllers for driving electro-opticdisplays having a plurality of pixels, each of which is capable ofdisplaying at least three gray levels. The present invention may ofcourse be applied to electro-optic displays having a greater number ofgray levels, for example 4, 8, 16 or more.

[0066] Also as already mentioned, driving bistable electro-opticdisplays requires very different methods from those normally used todrive liquid crystal displays (“LCD's”). In a conventional(non-cholesteric) LCD, applying a specific voltage to a pixel for asufficient period will cause the pixel to attain a specific gray level.Furthermore, the LC material is only sensitive to the magnitude of theelectric field, not its polarity. In contrast, bistable electro-opticdisplays act as impulse transducers, so there is no one-to-one mappingbetween applied voltage and gray state attained; the impulse (and thusthe voltage) which must be applied to a pixel to achieve a given graystate varies with the “initial” gray state of the relevant pixel.Furthermore, since bistable electro-optic displays need to be driven inboth directions (white to black, and black to white) it is necessary tospecify both the polarity and the magnitude of the impulse needed.

[0067] At this point, it is considered desirable to define certain termswhich are used herein in accordance with their conventional meaning inthe display art. Most of the discussion below will concentrate upon oneor more pixels of a display undergoing a single gray scale transition(i.e., a change from one gray level to another) from an “initial” stateto a “final” state. Obviously, the initial state and the final state areso designated only with regard to the single transition being consideredand in most cases the pixel with have undergone transitions prior to the“initial” state and will undergo further transitions after the “final”state. As explained below, some embodiments of the invention takeaccount not only of the initial and final states of the pixel but alsoof “prior” states, in which the pixel existed prior to achieving theinitial state. Where it is necessary to distinguish between multipleprior states, the term “first prior state” will be used to refer to thestate in which the relevant pixel existed one (non-zero) transitionprior to the initial state, the term “second prior state” will be usedto refer to the state in which the relevant pixel existed one (non-zero)transition prior to the first prior state, and so on. The term “non-zerotransition” is used to refer to a transition which effects a change ofat least one unit in gray scale; the term “zero transition” may be usedto refer to a “transition” which effects no change in gray scale of theselected pixel (although other pixels of the display may be undergoingnon-zero transitions at the same time).

[0068] As will readily be apparent to those skilled in image processing,a simple embodiment of the method of the present invention may takesaccount of only of the initial state of each pixel and the final state,and in such a case the look-up table will be two-dimensional. However,as already mentioned, some electro-optic media display a memory effectand with such media it is desirable, when generating the output signal,to take into account not only the initial state of each pixel but also(at least) the first prior state of the same pixel, in which case thelook-up table will be three-dimensional. In some cases, it may bedesirable to take into account more than one prior state of each pixel,thus resulting in a look-up table having four (if only the first andsecond prior states are taken into account) or more dimensions.

[0069] From a formal mathematical point of view, the present inventionmay be regarded as comprising an algorithm that, given information aboutthe initial, final and (optionally) prior states of an electro-opticpixel, as well as (optionally—see more detailed discussion below)information about the physical state of the display (e.g., temperatureand total operating time), will produce a function V(t) which can beapplied to the pixel to effect a transition to the desired final state.From this formal point of view, the controller of the present inventionmay be regarded as essentially a physical embodiment of this algorithm,the controller serving as an interface between a device wishing todisplay information and an electro-optic display.

[0070] Ignoring the physical state information for the moment, thealgorithm is, in accordance with the present invention, encoded in theform of a look-up table or transition matrix. This matrix will have onedimension each for the desired final state, and for each of the otherstates (initial and any prior states) are used in the calculation. Theelements of the matrix will contain a function V(t) that is to beapplied to the electro-optic medium.

[0071] The elements of the look-up table or transition matrix may have avariety of forms. In some cases, each element may comprise a singlenumber. For example, an electro-optic display may use a high precisionvoltage modulated driver circuit capable of outputting numerousdifferent voltages both above and below a reference voltage, and simplyapply the required voltage to a pixel for a standard, predeterminedperiod. In such a case, each entry in the look-up table could simplyhave the form of a signed integer specifying which voltage is to beapplied to a given pixel. In other cases, each element may comprise aseries of numbers relating to different portions of a waveform. Forexample, there are described below embodiments of the invention whichuse single- or double-prepulse waveforms, and specifying such a waveformnecessarily requires several numbers relating to different portions ofthe waveform. Also described below is an embodiment of the inventionwhich in effect applies pulse length modulation by applying apredetermined voltage to a pixel during selected ones of a plurality ofsub-scan periods during a complete scan. In such an embodiment, theelements of the transition matrix may have the form of a series of bitsspecifying whether or not the predetermined voltage is to be appliedduring each sub-scan period of the relevant transition. Finally, asdiscussed in more detail below, in some cases, such as atemperature-compensated display, it may be convenient for the elementsof the look-up table to be in the form of functions (or, in practice,more accurately coefficients of various terms in such functions).

[0072] It will be apparent that the look-up tables used in someembodiments of the invention may become very large. To take an extremeexample, consider a process of the invention for a 256 (2⁸) gray leveldisplay using an algorithm that takes account of initial, final and twoprior states. The necessary four-dimensional look-up table has 2³²entries. If each entry requires (say) 64 bits (8 bytes), the total sizeof the look-up table would be approximately 32 Gbyte. While storing thisamount of data poses no problems on a desktop computer, it may presentproblems in a portable device. However, in practice the size of suchlarge look-up tables can be substantially reduced. In many instances, ithas been found that there are only a small number of types of waveformsneeded for a large number of different transitions, with, for example,the length of individual pulses of a general waveform being variedbetween different transitions. Consequently, the length of individualentries in the look-up table can be reduced by making each entrycomprises (a) a pointer to an entry in a second table specifying one ofa small number of types of waveform to be used; and (b) a small numberof parameters specifying how this general waveform should be varied forthe relevant transition.

[0073] The values for the entries in the look-up table may be determinedin advance through an empirical optimization process. Essentially, onesets a pixel to the relevant initial state, applies an impulse estimatedto approximately equal that needed to achieve the desired final stateand measures the final state of the pixel to determine the deviation, ifany, between the actual and desired final state. The process is thenrepeated with a modified impulse until the deviation is less than apredetermined value, which may be determined by the capability of theinstrument used to measure the final state. In the case of methods whichtake into account one or more prior states of the pixel, in addition tothe initial state, it will generally be convenient to first determinethe impulse needed for a particular transition when the state of thepixel is constant in the initial state and all preceding states used indetermining the impulse, and then to “fine tune” this impulse to allowfor differing previous states.

[0074] The present method desirably provides for modification of theimpulse to allow for variation in temperature and/or total operatingtime of the display; compensation for operating time may be requiredbecause some electro-optic media “age” and their behavior changes afterextended operation. Such modification may be done in one of two ways.Firstly, the look-up table may be expanded by an additional dimensionfor each variable that is to be taken into account in calculating theoutput signal. Obviously, when dealing with continuous variables such astemperature and operating, it is necessary to quantize the continuousvariable in order to maintain the look-up table at a practicable finitesize. In order to find the waveform to be applied to the pixel, thecalculation means may simply choose the look-up table entry for thetable closest to the measured temperature. Alternatively, to providemore accurate temperature compensation, the calculation means may lookup the two adjacent look-up table entries on either side of the measuredcontinuous variable, and apply an appropriate interpolation algorithm toget the required entry at the measured intermediate value of thevariable. For example, assume that the matrix includes entries fortemperature in increments of 10° C. If the actual temperature of thedisplay is 25° C., the calculation would look up the entries for 20° and30 C., and use a value intermediate the two. Note that since thevariation of characteristics of electro-optic media with temperature isoften not linear, the set of temperatures for which the look-up tablestores entries may not be distributed linearly; for example, thevariation of many electro-optic media with temperature is most rapid athigh temperatures, so that at low temperatures intervals of 20° C.between look-up tables, might suffice, whereas at high temperaturesintervals of 5° C. might be desirable.

[0075] An alternative method for temperature/operating time compensationis to use look-up table entries in the form of functions of the physicalvariable(s), or perhaps more accurately coefficients of standard termsin such functions. For simplicity consider the case of a display whichuses a time modulation drive scheme in which each transition is handledby applying a constant voltage (of either polarity) to each pixel for avariable length of time, so that, absent any correction forenvironmental variables, each entry in the look-up table could consistonly of a single signed number representing the duration of time forwhich the constant voltage is to be applied, and its polarity. If it isdesired to correct such a display for variations in temperature suchthat the time T_(t) for which the constant voltage needs to be appliedfor a specific transition at a temperature t is given by:

T _(t) =T ₀ +AΔt+B(Δt)²

[0076] where T₀ is the time required at some standard temperature,typically the mid-point of the intended operating temperature range ofthe display, and Δt is the difference between t and the temperature atwhich T₀ is measured, the entries in the look-up table can consist ofthe values of T₀, A and B for the specific transition to which a givenentry relates, and the calculation means can use these coefficients tocalculate T_(t) at the measured temperature. To put it more generally,the calculation means finds the appropriate look-up table entry for therelevant initial and final states, then uses the function defined bythat entry to calculate the proper output signal having regard to theother variables to be taken into account.

[0077] The relevant temperature to be used for temperature compensationcalculations is that of the electro-optic material at the relevantpixel, and this temperature may differ significantly from ambienttemperature, especially in the case of displays intended for outdoor usewhere, for example, sunlight acting through a protective front sheet maycause the temperature of the electro-optic layer to be substantiallyhigher than ambient. Indeed, in the case of large billboard-type outdoorsigns, the temperature may vary between different pixels of the samedisplay if, for example, part of the display falls within the shadow ofan adjacent building, while the reminder is in full sunlight.Accordingly, it may be desirable to embed one or more thermocouples orother temperature sensors within or adjacent to the electro-optic layerto determine the actual temperature of this layer. In the case of largedisplays, it may also be desirable to provide for interpolation betweentemperatures sensed by a plurality of temperature sensors to estimatethe temperature of each particular pixel. Finally, in the case of largedisplays formed from a plurality of modules which can replacedindividually, the method and controller of the invention may provide fordifferent operating times for pixels in different modules.

[0078] The method and controller of the present invention may also allowfor the residence time (i.e., the period since the pixel last underwenta non-zero transition) of the specific pixel being driven. It has beenfound that, at least in some cases, the impulse necessary for a giventransition various with the residence time of a pixel in its opticalstate. Thus, it may be desirable or necessary to vary the impulseapplied for a given transition as a function of the residence time ofthe pixel in its initial optical state. In order to accomplish this, thelook-up table may optionally contain an additional dimension, which isindexed by a counter indicating the residence time of the pixel in itsinitial optical state. In addition, the controller will require anadditional storage area that contains a counter for every pixel in thedisplay. It will also require a display clock, which increments by onethe counter value stored in each pixel at a set interval. The length ofthis interval must be an integral multiple of the frame time of thedisplay, and therefore must be no less than one frame time. The size ofthis counter and the clock frequency will be determined by the length oftime over which the applied impulse will be varied, and the necessarytime resolution. For example, storing a 4-bit counter for each pixelwould allow the impulse to vary at 0.25 second intervals over a 4-secondperiod (4 seconds*4 counts/sec=16 counts=4 bits). The counter mayoptionally be reset upon the occurrence of certain events, such as thetransition of the pixel to a new state. Upon reaching its maximum value,the counter may be configured to either “roll over” to a count of zero,or to maintain its maximum value until it is reset.

[0079] The look-up table method of the present invention may of coursebe modified to take account of any other physical parameter which has adetectable effect upon the impulse needed to effect any one or morespecific transitions of an electro-optic medium. For example, the methodcould be modified to incorporate corrections for ambient humidity if theelectro-optic medium is found to be sensitive to humidity.

[0080] For a bistable electro-optic medium, the look-up table will havethe characteristic that, for any zero transition in which the initialand final states of the pixel are the same, the entry will be zero, orin other words, no voltage will be applied to the pixel. As a corollary,if no pixels on the display change during a given interval, then noimpulses must be applied. This enables ultra-low power operation, aswell as ensuring that the electro-optic medium is not overdriven while astatic image is being displayed. In general, the look-up table shouldonly retain information about non-null transitions. In other words, fortwo images, I and I+1, if a given pixel is in the same state in I andI+1, then state I+1 would not be stored in the prior state table, andthat no further information will be stored until that pixel undergoes atransition.

[0081] As will readily be apparent to those skilled in modern electronictechnology, the controller of the present invention can have a varietyof physical forms. and may use any conventional data processingcomponents. For example, the present method could be practiced using ageneral purpose digital computer in conjunction with appropriateequipment (for example, one or more digital analog converters, “DAC's”)to convert the digital outputs from the computer to appropriate voltagesfor application to pixels. Alternatively, the present method could bepracticed using an application specific integrated circuit (ASIC). Inparticular, the controller of the present invention could have the formof a video card which could be inserted into a personal computer toenable the images generated by the computer to be displayed on anelectro-optic screen instead of or in addition to an existing screen,such as a LCD. Since the construction of the controller of the presentinvention is well within the level of skill in the image processing art,it is unnecessary to describe its circuitry in detail herein.

[0082] A preferred physical embodiment of the controller of the presentinvention is a timing controller integrated circuit (IC). This ICaccepts incoming image data and outputs control signals to a collectionof data and select driver IC's, in order to produce the proper voltagesat the pixels to produce the desired image. This IC may accept the imagedata through access to a memory buffer that contains the image data, orit may receive a signal intended to drive a traditional LCD panel, fromwhich it can extract the image data. It may also receive any serialsignal containing information that it requires to perform the necessaryimpulse calculations. Alternately, this timing controller can beimplemented in software, or incorporated as a part of the CPU. Thetiming controller may also have the ability to measure any externalparameters that influence the operation of the display, such astemperature.

[0083] The controller can operate as follows. The look-up table(s) arestored in memory accessible to the controller. For each pixel in turn,all of the necessary initial, final and (optionally) prior and physicalstate information is supplied as inputs. The state information is thenused to compute an index into the look-up table. In the case ofquantized temperature or other correction, the return value from thislook-up will be one voltage, or an array of voltages versus time. Thecontroller will repeat this process for the two bracketing temperaturesin the look-up table, then interpolate between the values. For thealgorithmic temperature correction, the return value of the look-up willbe one or more parameters, which can then be inserted into an equationalong with the temperature, to determine the proper form of the driveimpulse, as already described. This procedure can be accomplishedsimilarly for any other system variables that require real-timemodification of the drive impulse. One or more of these system variablesmay be determined by, for example, the value of a programmable resistor,or a memory location in an EPROM, which is set on the display panel atthe time of construction in order to optimize the performance of thedisplay.

[0084] An important feature of the display controller is that, unlikemost displays, in most practical cases several complete scans of thedisplay will be required in order to complete an image update. Theseries of scans required for one image update should be considered to bean uninterruptible unit. If the display controller and image source areoperating asynchronously, then the controller must ensure that the databeing used to calculate applied impulses remains constant across allscans. This can be accomplished in one of two ways. Firstly, theincoming image data could be stored in a separate buffer by the displaycontroller (alternatively, if the display controller is accessing adisplay buffer through dual-ported memory, it could lock out access fromthe CPU). Secondly, on the first scan, the controller may store thecalculated impulses in an impulse buffer. The second option has theadvantage that the overhead for scanning the panel is only incurred onceper transition, and the data for the remaining scans can be outputdirectly from the buffer.

[0085] Optionally, imaging updating may be conducted in an asynchronousmanner. Although it will, in general, take several scans to effect acomplete transition between two images, individual pixels can begintransitions, or reverse transitions that have already started, inmid-frame. In order to accomplish this, the controller must keep trackof what portion of the total transition have been accomplished for agiven pixel. If a request is received to change the optical state of apixel that is not currently in transition, then the counter for thatpixel can be set to zero, and the pixel will begin transitioning on thenext frame. If the pixel is actively transitioning when a new request isreceived, then the controller will apply an algorithm to determine howto reach the new state from the current mid-transition state. For 1-bitgeneral image flow, one potential algorithm is simply to apply a pulseof reverse polarity, with amplitude and duration equal to the portion ofthe forward pulse that has already been applied.

[0086] In order to minimize the power necessary to operate a display,and to maximize the image stability of the electro-optic medium, thedisplay controller may stop scanning the display and reduce the voltageapplied to all pixels to, or close to, zero, when there are no pixels inthe display that are undergoing transitions. Very advantageously, thedisplay controller may turn off the power to its associated row andcolumn drivers while the display is in such a “hold” state, thusminimizing power consumption. In this scheme, the drivers would bereactivated when the next pixel transition is requested.

[0087]FIG. 1 of the accompanying drawings shows schematically anapparatus of the invention in use, together with associated apparatus.The overall apparatus (generally designated 10) shown in FIG. 1comprises an image source, shown as a personal computer 12 which outputson a data line 14 data representing an image. The data line 14 can be ofany conventional type and may be a single data line or a bus; forexample, the data line 14 could comprise a universal serial bus (USB),serial, parallel, IEEE-1394 or other line. The data which are placed onthe line 14 can be in the form of a conventional bit mapped image, forexample a bit map (BMP), tagged image file format (TIF), graphicsinterchange format (GIF) or jooint Photographic Experts Group (JPEG)file. Alternatively, however, the data placed on the line 14 could be inthe form of signals intended for driving a video device; for example,many computers provide a video output for driving an external monitorand signals on such outputs may be used in the present invention. Itwill be apparent to those skilled in imaging processing that theapparatus of the present invention described below may have to performsubstantial file format conversion and/or decoding to make use of thedisparate types of input signals which can be used, but such conversionand/or decoding is well within the level of skill in the art, andaccordingly, the apparatus of the present invention will be describedonly from the point at which the image data used as its original inputshave been converted to a format in which they can be processed by theapparatus.

[0088] The data line 14 extends to a controller unit 16 of the presentinvention, as described in detail below. This controller unit 16generates one set of output signals on a data bus 18 and a second set ofsignals on a separate data bus 20. The data bus 18 is connected to tworow (or gate) drivers 22, while the data bus 20 is connected to aplurality of column (or source) drivers 24. (The number of columndrivers 24 is greatly reduced in FIG. 1 for ease of illustration.) Therow and column drivers control the operation of a bistable electro-opticdisplay 26.

[0089] The apparatus shown in FIG. 1 is chosen to illustrate the variousunits used, and is most suitable for a developmental, “breadboard” unit.In actual commercial production, the controller 16 will typically bepart of the same physical unit as the display 26, and the image sourcemay also be part of this physical unit, as in conventional laptopcomputers equipped with LCD's, and in personal digital assistants. Also,the present invention is illustrated in FIG. 1 and will be mainlydescribed below, in conjunction with an active matrix displayarchitecture which has a single common, transparent electrode (not shownin FIG. 1) on one side of the electro-optic layer, this common electrodeextending across all the pixels of the, display. Typically, this commonelectrode lies between the electro-optic layer and the observer andforms a viewing surface through which an observer views the display. Onthe opposed side of the electro-optic layer is disposed a matrix ofpixel electrodes arranged in rows and columns such that each pixelelectrode is uniquely defined by the intersection of a single row and asingle column. Thus, the electric field experienced by each pixel of theelectro-optic layer is controlled by varying the voltage applied to theassociated pixel electrode relative to the voltage (normally designated“Vcom”) applied to the common front electrode. Each pixel electrode isassociated with at least one transistor, typically a thin filmtransistor. The gates of the transistors in each row are connected via asingle elongate row electrode to one of the row drivers 22. The sourceelectrodes of the transistors in each column are connected via a singleelongate column electrode to one of column drivers 24. The drainelectrode of each transistor is connected directly to the pixelelectrode. It will be appreciated that the assignment of the gates torows and the source electrodes to columns is arbitrary, and could bereversed, as could the assignment of source and drain electrodes.However, the following description will assume the conventionalassignments.

[0090] During operation, the row drivers 22 apply voltages to the gatessuch that the transistors in one and only one row are conductive at anygiven time. Simultaneously, the column drivers 24 apply predeterminedvoltages to each of the column electrodes. Thus, the voltages applied tothe column drivers are applied to only one row of the pixel electrodes,thus writing (or at least partially writing) one line of the desiredimage on the electro-optic medium. The row driver then shifts to makethe transistors in the next row conductive, a different set of voltagesare applied to the column electrodes, and the next line of the image iswritten.

[0091] It is emphasized that the present invention is not confined tosuch active matrix displays. Once the correct waveforms for each pixelof the image have been determined in accordance with the presentinvention, any switching scheme may be used to apply the waveforms tothe pixels. For example, the present invention can use a so-called“direct drive” scheme, in which each pixel is provided with a separatedrive line. In principle, the present invention can also use a passivematrix drive scheme of the type used in some LCD's, but it should benoted that, since many bistable electro-optic media lack a threshold forswitching (i.e., the media will change optical state if even a smallelectric field is applied for a prolonged period), such media areunsuitable for passive matrix driving. However, since it appears thatthe present invention will find its major application in active matrixdisplays, it will be described herein primarily with reference to suchdisplays.

[0092] The controller unit 16 (FIG. 1) has two main functions. Firstly,using the method of the present invention, the controller calculates atwo-dimensional matrix of impulses (or waveforms) which must be appliedto the pixels of a display to change an initial image to a final image.Secondly, the controller 16 calculates, from this matrix of impulses,all the timing signals necessary to provide the desired impulses at thepixel electrodes using the conventional drivers designed for use withLCD's to drive a bistable electro-optic display.

[0093] As shown in FIG. 2, the controller unit 16 shown in FIG. 1 hastwo main sections, namely a frame buffer 16A, which buffers the datarepresenting the final image which the controller 16B is to write to thedisplay 26 (FIG. 1), and the controller proper, denoted 16B. Thecontroller 16B reads data from the buffer 16A pixel by pixel andgenerates various signals on the data buses 18 and 20 as describedbelow.

[0094] The signals shown in FIG. 2 are as follows:

[0095] D0:D5—a six-bit voltage value for a pixel (obviously, the numberof bits in this signal may vary depending upon the specific row andcolumn drivers used)

[0096] POL—pixel polarity with respect to Vcom (see below)

[0097] START—places a start bit into the column driver 24 to enableloading of pixel values

[0098] HSYNC—horizontal synchronization signal, which latches the columndriver

[0099] PCLK—pixel clock, which shifts the start bit along the row driver

[0100] VSYNC—vertical synchronization signal, which loads a start bitinto the row driver

[0101] OE—output enable signal, which latches the row driver.

[0102] Of these signals, VSYNC and OE supplied to the row drivers 22 areessentially the same as the corresponding signals supplied to the rowdrivers in a conventional active matrix LCD, since the manner ofscanning the rows in the apparatus shown in FIG. 1 is in principleidentical to the manner of scanning an LCD, although of course the exacttiming of these signals may vary depending upon the preciseelectro-optic medium used. Similarly, the START, HSYNC and PCLK signalssupplied to the column drivers are essentially the same as thecorresponding signals supplied to the column drivers in a conventionalactive matrix LCD, although their exact timing may vary depending uponthe precise electro-optic medium used. Hence, it is considered that nofurther description of these output signals in necessary.

[0103]FIG. 3 illustrates, in a highly schematic manner, the way in whichthe controller 16B shown in FIG. 2 generates the D0:D5 and POL signals.As described above, the controller 16B stores data representing thefinal image 120 (the image which it is desired to write to the display),the initial image 122 previously written to the display, and optionallyone or more prior images 123 which were written to the display beforethe initial image. The embodiment of the invention shown in FIG. 3stores two such prior images 123. (Obviously, the necessary data storagecan be within the controller 16B or in an external data storage device.)The controller 16B uses the data for a specific pixel (illustrated asthe first pixel in the first row, as shown by the shading in FIG. 3) inthe initial, final and prior images 120. 122 and 123 as pointers into alook-up table 124, which provides the value of the impulse which must beapplied to the specific pixel to change the state of that pixel to thedesired gray level in the final image. The resultant output from thelook-up table 124, and the output from a frame counter 126, are suppliedto a voltage v. frame array 128, which generates the D0:D5 and POLsignals.

[0104] The controller 16B is designed for use with a TFT LCD driver thatis equipped with pixel inversion circuitry, which ordinarily alternatesthe polarity of neighboring pixels with respect to the top plane.Alternate pixels will be designated as even and odd, and are connectedto opposing sides of the voltage ladder. Furthermore, a driver input,labeled “polarity”, serves to switch the polarity of the even and oddpixels. The driver is provided with four or more gamma voltage levels,which can be set to determine the local slope of the voltage-levelcurve. A representative example of a commercial integrated circuit (IC)with these features is the Samsung KS0652 300/309 channel TFT-LCD sourcedriver. As previously discussed, the display to be driven uses a commonelectrode on one side of the electro-optic medium, the voltage appliedto this common electrode being referred to as the “top plane voltage” or“Vcom”.

[0105] In one embodiment, illustrated in FIG. 4 of the accompanyingdrawings, the reference voltages of the driver are arranged so that thetop plane voltage is placed at one half the maximum voltage (Vmax) whichthe driver can supply, i.e.

Vcom=Vmax/2

[0106] and the gamma voltages are arranged to vary linearly above andbelow the top plane voltage. (FIGS. 4 and 5 are drawn assuming an oddnumber of gamma voltages so that, for example, in FIG. 4 the gammavoltage VGMA(n/2+½) is equal to Vcom. If an even number of gammavoltages are present, both VGMA(n/2) and VGMA(n/2+1) are set equal toV_(com). Similarly, in FIG. 5, if an even number of gamma voltages arepresent, both VGMA(n/2) and VGMA(n/2+1) are set equal to the groundvoltage Vss.) The pulse length necessary to achieve all neededtransitions is determined by dividing the largest impulse needed tocreate the new image by Vmax/2. This impulse can be converted into anumber of frames by multiplying by the scan rate of the display. Thenecessary number of frames is then multiplied by two, to give an equalnumber of even and odd frames. These even and odd frames will correspondto whether the polarity bit is set high or low for the frame. For eachpixel in each frame, the controller 16B must apply an algorithm whichtakes as its inputs (1) whether the pixel is even or odd; (2) whetherthe polarity bit is high or low for the frame being considered; (3)whether the desired impulse is positive or negative; and (4) themagnitude of the desired impulse. The algorithm then determines whetherthe pixel can be addressed with the desired polarity during that frame.If so, the proper drive voltage (impulse/pulse length) is applied to thepixel. If not, then the pixel is brought to the top plane voltage(Vmax/2) to place it in a hold state, in which no electric field isapplied to the pixel during that frame.

[0107] For example, consider two neighboring pixels in the display, anodd pixel 1 and an even pixel 2. Further, assume that when the polaritybit is high, the odd pixels will be able to access the positive drivevoltage range (i.e. above the top plane voltage), and the even pixelswill be able to access the negative voltages (i.e. below the top planevoltage ). If both pixels 1 and 2 need to be driven with a positiveimpulse, then the following sequence must occur:

[0108] (a) during the positive polarity frames, pixel 1 is driven with apositive voltage, and pixel 2 is held at the top plane voltage; and

[0109] (b) during the negative polarity frames, pixel 1 is held at thetop plane voltage, while pixel 2 is driven with a positive voltage.

[0110] Although typically frames with positive and negative polaritywill be interleaved 1:1 (i.e., will alternate with each other), but thisis not necessary; for example, all the odd frames could be groupedtogether, followed by all the even frames. This would result inalternate columns of the display being driven in two separate groups.

[0111] The major advantage of this embodiment is that the common frontelectrode does not have to be switched during operation. The primarydisadvantage is that the maximum drive voltage available to theelectro-optic medium is only half of the maximum voltage of the driver,and that each line may only be driven 50% of the time. Thus. the refreshtime of such a display is four times the switching time of theelectro-optic medium under the same maximum drive voltage.

[0112] In a second embodiment of this form of the invention, the gammavoltages of the driver are arranged as shown in FIG. 5, and the commonelectrode switches between V=0 and V=Vmax. Arranging the gamma voltagesin this way allows both even and odd pixels to be driven simultaneouslyin a single direction, but requires that the common electrode beswitched to access the opposite drive polarity. In addition, becausethis arrangement is symmetric about the top plane voltage, a particularinput to the drivers will result in the same voltage being applied oneither an odd or an even pixel. In this case, the inputs to thealgorithm are the magnitude and sign of the desired impulse, and thepolarity of the top plane. If the current common electrode settingcorresponds to the sign of the desired impulse, then this value isoutput. If the desired impulse is in the opposite direction, then thepixel is set to the top plane voltage so that no electric field isapplied to the pixel during that frame.

[0113] As in the embodiment previously described, in this embodiment thenecessary length of the drive pulse can be calculated by dividing themaximum impulse by the maximum drive voltage, and this value convertedinto frames by multiplying by the display refresh rate. Again, thenumber of frames must be doubled, to account for the fact that thedisplay can only be driven in one direction with respect to the topplane at a time.

[0114] The major advantage of this second embodiment is that the fullvoltage of the driver can be used, and all of the outputs can be drivenat once. However, two frames are required for driving in opposeddirections. Thus. the refresh time of such a display is twice theswitching time of the electro-optic medium under the same maximum drivevoltage. The major drawback is the need to switch the common electrode,which may result in unwanted voltage artifacts in the electro-opticmedium, the transistors associated with the pixel electrodes, or both.

[0115] In either embodiment, the gamma voltages are normally arranged ona linear ramp between the maximum voltages of the driver and the topplane voltage. Depending upon the design of the driver, it may benecessary to set one or more of the gamma voltages at the top planevalue, in order to ensure that the driver can actually produce the topplane voltage on the output.

[0116] Reference has already been made above to the need to adapt themethod of the present invention to the limitations of conventionaldrivers designed for use with LCD's. More specifically, conventionalcolumn drivers for LCD's, and particularly super twisted nematic (STN)LCD's (which can usually handle higher voltages than other types ofcolumn drivers), are only capable of applying one of two voltages to adrive line at any given time, since this is all that apolarity-insensitive LC material requires. In contrast, to drivepolarity-sensitive electro-optic displays, a minimum of three drivervoltage levels are necessary. The three driver voltages required are V−,which drives a pixel negative with respect to the top plane voltage, V+,which drives a pixel positive with respect to the top plane voltage, and0V with respect to the top plane voltage, which will hold the pixel inthe same display state.

[0117] The method of the present invention can, however, be practicedwith this type of conventional LCD driver, provided that the controlleris arranged to apply an appropriate sequence of voltages to the inputsof one or more column drivers, and their associated row drivers, inorder to apply the necessary impulses to the pixels of an electro-opticdisplay.

[0118] There are two principal variants of this approach. In the firstvariant, all the impulses applied must have one of three values: +I, −Ior 0, where:

+I=−(−I)=Vapp*t _(pulse)

[0119] where Vapp is the applied voltage above the top plane voltage,and t_(pulse) is the pulse length in seconds. This variant only allowsthe display to operate in a binary (black/white) mode. In the secondvariant, the applied impulses may vary from +I to −I, but must beintegral multiples of Vapp/freq, where freq is the refresh frequency ofthe display.

[0120] This aspect of the present invention takes advantage of the factthat, as already noted, conventional LCD drivers are designed to reversepolarity at frequent intervals to avoid certain undesirable effectswhich might otherwise be produced in the display. Consequently, suchdrivers are arranged to receive from the controller a polarity orcontrol voltage, which can either be high or low. When a low controlvoltage is asserted, the output voltage on any given driver output linecan adopt one of two out of the possible three voltages required, say V1or V2, while when a high control voltage is asserted, the output voltageon any given line can adopt one of a different two of the possible threevoltages required, say V2 or V3. Thus, while only two out of the threerequired voltages can be addressed at any specific time, all threevoltages can be achieved at differing times. The three required voltageswill usually satisfy the relationship:

V2=(V3+V1)/2

[0121] and V1 may be at or near the logic ground.

[0122] In this method of the invention, the display will be scanned2*t_(pulse)*freq times. For half these scans (i.e., for t_(pulse)*freqscans), the driver will be set to output either V1 or V2, which willnormally be equal to −V and Vcom, respectively. Thus, during thesescans, the pixels are either driven negative, or held in the samedisplay state. For the other half of the scans, the driver will beswitched to output either V2 or V3, which will normally be at Vcom and+V respectively. In these scans, the pixels are driven positive or heldin the same display state. Table 1 below illustrates how these optionscan be combined to produce a drive in either direction or a hold state;the correlation of positive driving with approach to a dark state andnegative driving with approach to a light state is of course a functionof the specific electro-optic medium used. TABLE 1 Drive sequence forachieving bi-directional drive plus hold with STN drivers Driver outputsDesired Drive V1-V2 V2-V3 positive (drive dark) V2 V3 negative (drivewhite) V1 V2 hold V2 V2

[0123] There are several different ways to arrange the two portions ofthe drive scheme (i.e., the two different types of scans or “frames”).For example, the two types of frames could alternate. If this is done ata high refresh rate, then the electro-optic medium will appear to besimultaneously lightening and darkening, when in fact it is being drivenin opposed direction in alternate frames. Alternatively, all of theframes of one type could occur before any of the frames of the secondtype; this would result in a two-step drive appearance. Otherarrangements are of course possible; for example two or more frames ofone type followed by two or more of the opposed type. Additionally, ifthere are no pixels that need to be driven in one of the two directions,then the frames of that polarity can be dropped, reducing the drive timeby 50%.

[0124] While this first variant can only produce binary images, thesecond variant can render images with multiple gray scale levels. Thisis accomplished by combining the drive scheme described above withmodulation of the pulse widths for different pixels. In this case, thedisplay is again scanned 2*t_(pulse)*freq times, but the driving voltageis only applied to any particular pixel during enough of these scans toensure that the desired impulse for that particular pixel is achieved.For example, for each pixel, the total applied impulse could berecorded, and when the pixel reached its desired impulse, the pixelcould be held at the top plane voltage for all subsequent scans. Forpixels that need to be driven for less than the total scanning time, thedriving portion of this time (i.e., the portion of the time during whichan impulse is applied to change the display state of the pixel, asopposed to the holding portion during which the applied voltage simplymaintains the display state of the pixel) may be distributed in avariety of ways within the total time. For example, all driving portionscould be set to start at the beginning of the total time, or all drivingportions could instead be timed to complete at the end of the totaltime. As in the first variant, if at any time in the second variant nofurther impulses of a particular polarity need to be applied to anypixel, then the scans applying pulses of that polarity can beeliminated. This may mean that the entire pulse is shortened, forexample, if the maximum impulse to be applied in both the positive andnegative directions is less than the maximum allowable impulse.

[0125] To take a highly simplified case for purposes of illustration,consider the application of the gray scale scheme described above to adisplay having four gray levels, namely black (level 0), dark gray(level 1), light gray (level 2) and white (level 3). One possible drivescheme for such a display is summarized in Table 2 below. TABLE 2 FrameNo. 1 2 3 4 5 6 Parity Odd Even Odd Even Odd Even Transition 0-3 + 0 +0 + 0 0-2 + 0 + 0 0 0 0-1 + 0 0 0 0 0 0-0 0 0 0 0 0 0 3-0 0 − 0 − 0 −2-0 0 − 0 − 0 0 1-0 0 − 0 0 0 0

[0126] For ease of illustration, this drive scheme is assumed to useonly six frames although in practice a greater number would typically beemployed. These frames are alternately odd and even. White-goingtransitions (i.e., transitions in which the gray level is increased) aredriven only on the odd frames, while black-going transitions (i.e.,transitions in which the gray level is decreased) are driven only on theeven frames. On any frame when a pixel is not being driven, it is heldat the same voltage as the common front electrode, as indicated by “0”in Table 2. For the 0-3 (black-white) transition, a white-going impulseis applied (i.e., the pixel electrode is held at a voltage relative tothe common front electrode which tends to increase the gray level of thepixel) in each of the odd frames, Frames 1, 3 and 5. For a 0-2 (black tolight gray) transition, on the other hand, a white-going impulse isapplied only in Frames 1 and 3, and no impulse is applied in Frame 5;this is of course arbitrary, and, for example, a white-going impulsecould be applied in Frames 1 and 5 and no impulse applied in Frame 3.For a 0-1 (black to dark gray) transition, a white-going impulse isapplied only in Frame 1, and no impulse is applied in Frames 3 and 5;again, this is arbitrary, and, for example, a white-going impulse couldbe applied in Frame 3 and no impulse applied in Frames 1 and 5.

[0127] The black-going transitions are handled in a manner exactlysimilar to the corresponding white-going transitions except that theblack-going impulses are applied only in the even frames of the drivescheme. It is believed that those skilled in driving electro-opticdisplays will readily be able to understand the manner in which thetransitions not shown in Table 2 are handled from the foregoingdescription.

[0128] The sets of impulses described above can either be stand-alonetransitions between two images (as in general image flow), or they maybe part of a sequence of impulses designed to accomplish an imagetransition (as in a slide-show waveform).

[0129] Although emphasis has been laid above on methods of the presentinvention which permit the use of conventional drivers designed for usewith LCD's, the present invention can make use of custom drivers, and adriver which is intended to enable accurate control of gray states in anelectro-optic display, while achieving rapid writing of the display willnow be described with reference to FIGS. 6 and 7.

[0130] As already discussed, to first order, many electro-optic mediarespond to a voltage impulse, which can be expressed as V times t(ormore generally, by the integral of V with respect to t) where V is thevoltage applied to a pixel and t is the time over which the voltage isapplied. Thus, gray states can be obtained by modulating the length ofthe voltage pulse applied to the display, or by modulating the appliedvoltage, or by a combination of these two.

[0131] In the case of pulse width modulation in an active matrixdisplay, the attainable pulse width resolution is simply the inverse ofthe refresh rate of the display. In other words, for a display with a100 Hz refresh rate, the pulse length can be subdivided into 10 msintervals. This is because each pixel in the display is only addressedonce per scan, when the select line for the pixels in that row areactivated. For the rest of the time, the voltage on the pixel may bemaintained by a storage capacitor, as described in the aforementioned WO01/07961. As the response speed of the electro-optic medium becomesfaster, the slope of the reflectivity versus time curve becomes steeperand steeper. Thus, to maintain the same gray scale resolution, therefresh rate of the display must increase accordingly. Increasing therefresh rate results in higher power consumption, and eventually becomesimpractical as the transistors and drivers are expected to charge thepixel and line capacitance in a shorter and shorter time.

[0132] On the other hand, in a voltage modulated display, the impulseresolution is simply determined by the number of voltage steps, and isindependent of the speed of the electro-optic medium. The effectiveresolution can be increased by imposing a nonlinear spacing of thevoltage steps, concentrating them where the voltage/reflectivityresponse of the electro-optic medium is steepest.

[0133]FIG. 6 of the accompanying drawings is a schematic representationof the tradeoffs between the pulse width modulation (PWM) and voltagemodulation (VM) approaches. The horizontal axis represents pulse length,while the vertical axis represents voltage. The reflectivity of theparticle-based electrophoretic display as a function of these twoparameters is represented as a contour plot, with the bands and spacesrepresenting differences of 1 L* in the reflected luminance of thedisplay, where L* has the usual ICE definition:

L*=116(R/R ₀)^(1/3)−16

[0134] where R is the reflectance and R₀ is a standard reflectancevalue. (It has been found empirically that a difference in luminance of1 L* is just noticeable to an average subject in dual stimulusexperiments.) This particular particle-based electrophoretic medium usedin the experiments summarized in FIG. 6 had a response time of 200 ms atthe maximum voltage (16 V) shown in the Figure.

[0135] The effects of pulse width modulation alone can be determined bytraversing the plot horizontally along the top, while the effects ofvoltage modulation alone are seen by examining the right vertical edge.From this plot, it is clear that, if a display using this particularmedium were driven at a refresh rate of 100 Hz in a pulse widthmodulation (PWM) mode, it would not be possible to obtain a reflectivitywithin ±1 L* in the middle gray region, where the contours are steepest.In voltage modulation (VM) mode, achieving a reflectivity within ±1 L*would require 128 equally spaced voltage levels, while running at aframe rate as low as 5 Hz (assuming, of course, that the voltage holdingcapability of the pixel, provided by a capacitor, is high enough). Inaddition, these two approaches can be combined to achieve the sameaccuracy with fewer voltage levels. To further reduce the requirednumber of voltage levels, they could be concentrated in the steep middleportion of the curves shown in FIG. 6 but made sparse in the outerregions. This could be accomplished with a small number of input gammavoltages. To further reduce the required number of voltage levels, theycould be concentrated at advantageous values. For example, very smallvoltages are not useful for achieving transitions if application of sucha small voltage over the allotted address time is not sufficient to makeany of the desired gray state transitions. Choosing a distribution ofvoltages that excludes such small voltages allows the allowed voltagesto be more advantageously placed.

[0136] Since bistable electro-optic displays are sensitive to thepolarity of the applied electric field, as noted above, it is notdesirable to reverse the polarity of the drive voltage on successiveframes (images), as is usually done with LCD's, and frame, pixel andline inversion are unnecessary, and indeed counterproductive. Forexample, LCD drivers with pixel inversion deliver voltages ofalternating polarity in alternate frames. Thus, it is only possible todeliver an impulse of the proper polarity in one half of the frames.This is not a problem in an LCD, where the liquid crystal material innot sensitive to polarity, but in a bistable electro-optic display itdoubles the time required to address the electro-optic medium.

[0137] Similarly, because bistable electro-optic displays are impulsetransducers and not voltage transducers, the displays integrate voltageerrors over time, which can result in large deviations of the pixels ofthe display from their desired optical states. This makes it importantto use drivers with high voltage accuracy, and a tolerance of ±3 mV orless is recommended.

[0138] To enable a driver to address a monochrome XGA (1024×768) displaypanel at a 75 Hz refresh rate, a maximum pixel clock rate of 60 MHz isrequired; achieving this clock rate is within the state of the art.

[0139] As already mentioned, one of the primary virtues ofparticle-based electrophoretic and other similar bistable electro-opticdisplays is their image stability, and the consequent opportunity to runthe display at very low power consumption. To take maximum advantage ofthis opportunity, power to the driver should be disabled when the imageis not changing. Accordingly, the driver should be designed to powerdown in a controlled manner, without creating any spurious voltages onthe output lines. Because entering and leaving such a “sleep” mode willbe a common occurrence, the power-down and power-up sequences should beas rapid as possible, and should have minimal effects on the lifetime ofthe driver.

[0140] In addition, there should be an input pin that brings all of thedriver output pins to Vcom, which will hold all of the pixels at theircurrent optical state without powering down the driver.

[0141] The drivers of the present invention are useful, inter alia, fordriving medium to high resolution, high information content portabledisplays, for example a 7 inch (178 mm) diagonal XGA monochrome display.To minimize the number of integrated circuits required in such highresolution panels, it is desirable to use drivers with a high number(for example, 324) of outputs per package. It is also desirable that thedriver have an option to run in one or more other modes with fewer ofits outputs enabled. The preferred method for attaching the integratedcircuits to the display panel is tape carrier package (TCP), so it isdesirable to arrange the sizing and spacing of the driver outputs tofacilitate use of this method.

[0142] The present drivers will typically be used to drive small tomedium active matrix panels at around 30 V. Accordingly, the driversshould be capable of driving capacitative loads of approximately 100 pF.

[0143] A block diagram of a preferred driver (generally designated 200)of the invention is given in FIG. 7 of the accompanying drawings. Thisdriver 200 comprises a shift register 202, a data register 204, a datalatch 206, a digital to analogue converter (DAC) 208 and an outputbuffer 210. This driver differs from those typically used to drive LCD'sin that it provides for a polarity bit associated with each pixel of thedisplay, and for generating an output above or below the top planevoltage controlled by the relevant polarity bit.

[0144] The signal descriptions for this preferred driver are given inthe following Table 3: TABLE 3 Symbol Pin Name Description VDD Logicpower supply 2.7-3.6 V AVDD Driver power supply 10-30 V VSS Ground 0 VY1- Driver outputs, fed to the D/A converted 64 level Y324 columnelectrodes of the analog outputs display D0(0:5) Display data input, odddots 6 bit gray scale data for odd dots, D0:0 = least significant bit(LSB) D1(0:5) Display data input, even dots 6 bit gray scale data foreven dots, D1:0 LSB D0POL Odd dot polarity control input Determineswhich set of gamma voltages current odd dot will reference. D0POL = 1:odd dot will reference VGAM6-11 D0POL = 0: odd dot will referenceVGAM1-6 D1POL Even dot polarity Determines which set of control inputgamma voltages current even dot will reference. D1POL = 1: odd dot willreference VGAM6-11 D1POL = 0: odd dot will reference VGAM1-6 SHL Shiftdirection control input Controls shift direction in 162 bit shiftregister SHL = H: DIO1 input, Y1 −> Y324 SHL = L: DIO1 output, Y324 −>Y1 DIO1 Start pulse input/output SHL = H: Used as the start pulse inputpin SHL = L: Used as the start pulse output pin DIO2 Start pulseinput/output for SHL = H: Used as the start 256 lines pulse output pinfor 256 lines active SHL = L: Used as the start pulse input pin for 256lines, tie low if not used DIO3 Start pulse input/output for SHL = H:Used as the start 260 lines pulse output pin for 260 lines active SHL =L: Used as the start pulse input pin for 260 lines, tie low if not usedDIO4 Start pulse input/output for SHL = H: Used as the start 300 linespulse output pin for 300 lines active SHL = L: Used as the start pulseinput pin for 300 lines, tie low if not used DIO5 Start pulseinput/output for SHL = H: Used as the start 304 lines pulse output pinfor 304 lines active SHL = L: Used as the start pulse input pin for 304lines, tie low if not used DIO6 Start pulse input/output for SHL = H:Used as the start 320 lines pulse output pin for 320 lines active SHL =L: Used as the start pulse input pin for 320 lines, tie low if not usedDIO7 Start pulse input/output for SHL = H: Used as the start 324 linespulse output pin for 324 lines active SHL = L: Used as the start pulseinput pin for 324 lines, tie low if not used CLK1 Shift clock input Two6 bit gray values and two polarity control values for two display dotsare loaded at every rising edge CLK2 Latch input Latches the contents ofthe data register on a rising edge and transfers latched values to theD/A converter block. BL Blanking input (this does not Sets all outputsto VGAM6 actually blank the bistable level BL = H: All outputs setdisplay, but simply stops the to VGAM6 BL = L: All driver writing to thsdisplay, outputs reflect D/A values already written to remain) VGAM1-Lower gamma reference Determine grayscale voltage 6 voltages outputsthrough resistive DAC system VGAM6- Upper gamma reference Determinegrayscale voltage 11 voltages outputs through resistive DAC system

[0145] The driver 200 operates in the following manner. First, a startpulse is provided by setting (say) DIO1 high to reset the shift register202 to a starting location. (As will readily be apparent to thoseskilled in display driver technology, the various DIOx inputs to theshift register are provided to enable the driver to be used withdisplays having varying numbers of columns, and only one of these inputsis used with any given display, the others being tied permanently low.)The shift register now operates in the conventional manner used inLCD's; at each pulse of CLK1, one and only one of the 162 outputs of theshift register 202 goes high, the others being held low, and the highoutput being shifted one place at each pulse of CLK1. As schematicallyindicated in FIG. 7, each of the 162 outputs of the shift register 202is connected to two inputs of data register 204, one odd input and oneeven input.

[0146] The display controller (cf. FIG. 2) provides two six-bit impulsevalues D0(0:5) and D1(0:5) and two single-bit polarity signals D0POL andD1POL on the inputs of the data register 204. At the rising edge of eachclock pulse CLK1, two seven-bit numbers (D0POL+D0(0:5) andD1POL+D1(0:5)) are written into registers in data register 204associated with the selected (high) output of shift register 202. Thus,after 162 clock pulses CLK1, 324 seven-bit numbers (corresponding to theimpulse values for one complete line of the display for one frame) havebeen written into the 324 registers present in data register 204.

[0147] At the rising edge of each clock pulse CLK2, these 324 seven-bitnumbers are transferred from the data register 204 to the data latch206. The numbers thus placed in the data latch 206 are read by the DAC208 and, in conventional fashion, corresponding analogue values areplaced on the outputs of the DAC 208 and fed, via the buffer 210 to thecolumn electrodes of the display, where they are applied to pixelelectrodes of one row selected in conventional fashion by a row driver(not shown). It should be noted, however, that the polarity of eachcolumn electrode with respect to Vcom is controlled by the polarity bitDOPOL or DI POL written into the data latch 206 and thus thesepolarities do not alternate between adjacent column electrodes in theconventional manner used in LCD's.

[0148]FIG. 8 is a flow chart illustrating a program which may be run bythe controller unit shown in FIGS. 1 and 2. This program (generallydesignated 300) is intended for use with a look-up table method of theinvention (described in more detail below) in which all pixels of adisplay are erased and then re-addressed each time an image is writtenor refreshed.

[0149] The program begins with a “powering on” step 302 in which thecontroller is initialized, typically as a result of user input, forexample a user pushing the power button of a personal digital assistant(PDA). The step 302 could also be triggered by, for example, the openingof the case of a PDA (this opening being detected either by a mechanicalsensor or by a photodetector), by the removal of a stylus from its restin a PDA, by detection of motion when a user lifts a PDA, or by aproximity detector which detects when a user's hand approaches a PDA.

[0150] The next step 304 is a “reset” step in which all the pixels ofthe display are driven alternately to their black and white states. Ithas been found that, in at least some electro-optic media, such“flashing” of the pixels is necessary to ensure accurate gray statesduring the subsequent writing of an image on the display. It has alsobeen found that typically at least 5 flashes (counting each successiveblack and white state as one flash) are required, and in some casesmore. The greater the number of flashes, the more time and energy thatthis step consumes, and thus the longer the time that must elapse beforethe user can see a desired image upon the display. Accordingly, it isdesirable that the number of flashes be kept as small as possibleconsistent with accurate rendering of gray states in the imagesubsequently written. At the conclusion the reset step 304, all thepixels of the display are in the same black or white state.

[0151] The next step 306 is a writing or “sending out image” step inwhich the controller 16 sends out signals to the row and column drivers22 and 24 respectively (FIGS. 1 and 2) in the manner already described,thus writing a desired image on the display. Since the display isbistable, once the image has been written, it does not need to berewritten immediately, and thus after writing the image, the controllercan cause the row and column drivers to cease writing to the display,typically by setting a blanking signal (such as setting signal BL inFIG. 7 high).

[0152] The controller now enters a decision loop formed by steps 308,310 and 312. In step 308, the controller 16 checks whether the computer12 (FIG. 1) requires display of a new image. If so, the controllerproceeds, in an erase step 314 to erase the image written to the displayat step 306, thus essentially returning the display to the state reachedat the end of reset step 304. From erase step 314, the controllerreturns to step 304, resets as previously described, and proceeds towrite the new image.

[0153] If at step 308 no new image needs to be written to the display,the controller proceeds to a step 310, at which it determines when theimage has remained on the display for more than a predetermined period.As is well known to those skilled in display technology, images writtenon bistable media do not persist indefinitely, and the images graduallyfade (i.e., lose contrast). Furthermore, in some types of electro-opticmedium, especially electrophoretic media, there is often a trade-offbetween writing speed of the medium and bistability, in that media whichare bistable for hours or days have substantially longer writing timesthan media which are only bistable for seconds or minutes. Accordingly,although it is not necessary to rewrite the electro-optic mediumcontinuously, as in the case of LCD's, to provide an image with goodcontrast, it may be desirable to refresh the image at intervals of (say)a few minutes. Thus, at step 310 the controller determines whether thetime which has elapsed since the image was written at step 306 exceedssome predetermined refresh interval, and if so the controller proceedsto erase step 314 and then to reset step 304, resets the display aspreviously described, and proceeds to rewrite the same image to thedisplay.

[0154] (The program shown in FIG. 8 may be modified to make use of bothlocal and global rewriting, as discussed in more detail below. If so,step 310 may be modified to decide whether local or global rewriting isrequired. If, in this modified program, at step 310 the programdetermines that the predetermined time has not expired, no action istaken. If, however, the predetermined time has expired, step 310 doesnot immediately invoke erasure and rewriting of the image; instead step310 simply sets a flag (in the normal computer's sense of that term)indicating that the next image update should be effected globally ratherthan locally. The next time the program reaches step 306, the flag ischecked; if the flag is set, the image is rewritten globally and thenthe flag is cleared, but if the flag is not set, only local rewriting ofthe image is effected.)

[0155] If at step 310 it is determined that the refresh interval has notbeen exceeded, the controller proceeds to a step 312, where itdetermines whether it is time to shut down the display and/or the imagesource. In order to conserve energy in a portable apparatus, thecontroller will not allow a single image to be refreshed indefinitely,and terminates the program shown in FIG. 8 after a prolonged period ofinactivity. Accordingly, at step 310 the controller determines whether apredetermined “shut-down” period (greater than the refresh intervalmentioned above) has elapsed since a new image (rather than a refresh ofa previous image) was written to the display, and if so the programterminates, as indicated at 314. Step 314 may include powering down theimage source. Naturally, the user still has access to a slowly-fadingimage on the display after such program termination. If the shut-downperiod has not been exceeded, the controller proceeds from step 312 backto step 308.

[0156] Various possible waveforms for carrying out the look-up tablemethod of the present invention will now be described, though by way ofexample only. However, first some general considerations regardingwaveforms to be used in the present invention will be discussed.

[0157] Waveforms for bistable displays that exhibit the aforementionedmemory effect can be grouped into two major classes, namely compensatedand uncompensated. In a compensated waveform, all of the pulses areprecisely adjusted to account for any memory effect in the pixel. Forexample, a pixel undergoing a series of transitions through gray scalelevels 1-3-4-2 might receive a slightly different impulse for the 4-2transition than a pixel that undergoes a transition series 1-2-4-2. Suchimpulse compensation could occur by adjusting the pulse length, thevoltage, or by otherwise changing the V(t) profile of the pulses. In anuncompensated waveform, no attempt is made to account for any priorstate information (other than the initial state). In an uncompensatedwaveform, all pixels undergoing the 4-2 transition would receiveprecisely the same pulse. For an uncompensated waveform to worksuccessfully, one of two criteria must be met. Either the electro-opticmaterial must not exhibit a memory effect in its switching behavior, oreach transition must effectively eliminate any memory effect on thepixel.

[0158] In general, uncompensated waveforms are best suited for use withsystems capable of only coarse impulse resolution. Examples would be adisplay with tri-level drivers, or a display capable of only 2-3 bits ofvoltage modulation. A compensated waveform requires fine impulseadjustments, which are not possible with these systems. Obviously, whilea coarse-impulse system is preferably restricted to uncompensatedwaveforms, a system with fine impulse adjustment can implement eithertype of waveform.

[0159] The simplest uncompensated waveform is 1-bit general image flow(1-bit GIF). In 1-bit GIF, the display transitions smoothly from onepure black-and-white image to the next. The transition rule for thissequence can be stated simply: If a pixel is switching from white toblack, then apply an impulse I. If it is switching from black to white,apply the impulse of the opposite polarity, −I. If a pixel remains inthe same state, then no impulse is applied to that pixel. As previouslystated, the mapping of the impulse polarity to the voltage polarity ofthe system will depend upon the response function of the material.

[0160] Another uncompensated waveform that is capable of producinggrayscale images is the uncompensated n-prepulse slide show (n-PP SS).The uncompensated slide show waveform has three basic sections. First,the pixels are erased to a uniform optical state, typically either whiteor black. Next, the pixels are driven back and forth between two opticalstates, again typically white and black. Finally, the pixel is addressedto a new optical state, which may be one of several gray states. Thefinal (or writing) pulse is referred to as the addressing pulse, and theother pulses (the first (or erasing) pulse and the intervening (orblanking) pulses) are collectively referred to as prepulses. A waveformof this type will be described below with reference to FIGS. 9 and 10.

[0161] Prepulse slide show waveforms can be divided into two basicforms, those with an odd number of prepulses, and those with an evennumber of prepulses. For the odd-prepulse case, the erasing pulse may beequal in impulse and opposite in polarity to the immediately previouswriting pulse (again, see FIG. 9 and discussion thereof below). In otherwords, if the pixel is written to gray from black, the erasing pulsewill take the pixel back to the black state. In the even-prepulse case,the erasing pulse should be of the same polarity as the previous writingpulse, and the sum of the impulses of the previous writing pulse and theerasing pulse should be equal to the impulse necessary to fullytransition from black to white. In other words, if a pixel is writtenfrom black in the even-prepulse case, then it must be erased to white.

[0162] After the erasing pulse, the waveform includes either zero or aneven number of blanking pulses. These blanking pulses are typicallypulses of equal impulse and opposite polarity, arranged so that thefirst pulse is of opposite polarity to the erasing pulse. These pulseswill generally be equal in impulse to a full black-white pulse, but thisis not necessarily the case. It is also only necessary that pairs ofpulses have equal and opposite impulses it is possible that there may bepairs of widely varying impulses chained together, i.e. +I, −I, +0.1I,−0.1I, +4I, −4I.

[0163] The last pulse to be applied is the writing pulse. The impulse ofthis pulse is chosen based only upon the desired optical state (not uponthe current state, or any prior state). In general, but not necessarily,the pulse will increase or decrease monotonically with gray state value.Since this waveform is specifically designed for use with coarse impulsesystems, the choice of the writing pulse will generally involve mappinga set of desired gray states onto a small number of possible impulsechoices, e.g. 4 gray states onto 9 possible applied impulses.

[0164] Examination of either the even or odd form of the uncompensatedn-prepulse slide show waveform will reveal that the writing pulse alwaysbegins from the same direction, i.e. either from black or from white.This is an important feature of this waveform. Since the principle ofthe uncompensated waveform is that the pulse length can not becompensated accurately to ensure that pixels reach the same opticalstate, one cannot to expect to reach an identical optical state whenapproaching from opposite extreme optical states (black or white).Accordingly, there are two possible, polarities for either of theseforms, which can be labeled “from black” and “from white.”

[0165] One major shortcoming of this type of waveform is that it haslarge-amplitude optical flashes between images. This can be improved byshifting the update sequence by one superframe time for half of thepixels, and interleaving the pixels at high resolution, as discussedbelow with reference to FIGS. 9 and 10. Possible patterns include everyother row, every other column, or a checkerboard pattern. Note, thisdoes not mean using the opposite polarity, i.e. “from black” vs “fromwhite”, since this would result in non-matching gray scales onneighboring pixels. Instead, this can be accomplished by delaying thestart of the update by one “superframe” (a grouping of frames equivalentto the maximum length of a black-white update) for half of the pixels(i.e. the first set of pixels completes the erase pulse, then the secondset of pixels begin the erase pulse as the first set of pixels begin thefirst blanking pulse). This will require the addition of one superframefor the total update time, to allow for this synchronization.

[0166] It might at first appear that the ideal method of the presentinvention would be so-called “general grayscale image flow” in which thecontroller arranges each writing of an image so that each pixeltransitions directly from its initial gray level to its final graylevel. In practice, however, general grayscale image flow suffers froman accumulation of errors problem. The impulse applied in any givengrayscale transition will necessarily differ from that theoreticallynecessary because of facts such as unavoidable variations in thevoltages output by drivers, manufacturing variations in the thickness ofthe electro-optic medium, etc. Suppose that the average error on eachtransition, expressed in terms of the difference between the theoreticaland the actual reflectance of the display is ±0.2 L*. After 100successive transitions, the pixels will display an average deviationfrom their expected state of 2 L*; such deviations are apparent to theaverage observer on certain types of images. To avoid this problem, itmay be desirable to arrange the drive scheme used in the presentinvention so that any given pixel can only undergo a predeterminedmaximum number of gray scale transitions before passing through oneextreme optical state (black or white). These extreme optical states actas “rails” in that after a particular impulse has been applied to anelectro-optic medium, the medium cannot become any blacker or whiter.Thus, the next transition away from the extreme optical state can startfrom an accurately known optical state, in effect canceling out anypreviously accumulated errors. Various techniques for minimizing theoptical effects of such passage of pixels through extreme optical statesare discussed below.

[0167] A first, simple drive scheme useful in the present invention willnow be described with reference to a simple two-bit gray scale systemhaving black (level 0), dark gray (level 1), light gray (level 2) andwhite (level 3) optical states, transitions being effected using a pulsewidth modulation technique, and a look-up table for transitions as setout in Table 4 below. TABLE 4 Transition Impulse Transition Impulse 0-00 0-0 0 0-1 n 1-0 −n 0-2 2n 2-0 −2n 0-3 3n 3-0 −3n

[0168] where n is a number dependent upon the specific display, and −nindicates a pulse having the same length as a pulse n but of oppositepolarity. It will further be assumed that at the end of the reset pulse304 in FIG. 8 all the pixels of the display are black (level 0). Since,as described below, all transitions take place through an interveningblack state, the only transitions effected are those to or from thisgray state. Thus, the size of the necessary look-up table issignificantly reduced, and obviously the factor by which look-up tablesize is thus reduced increases with the number of gray levels of thedisplay.

[0169]FIG. 9 shows the transitions of one pixel associated with thedrive scheme of FIG. 8. At the beginning of the reset step 304, thepixel is in some arbitrary gray state. During the reset step 304, thepixel is driven alternately to three black states and two interveningwhite states, ending in its black state. The pixel is then, at 306,written with the appropriate gray level for a first image, assumed to belevel 1. The pixel remains at this level for some time during which thesame image is displayed; the length of this display period is greatlyreduced in FIG. 9 for ease of illustration. At some point, a new imageneeds to be written, and at this point, the pixel is returned to black(level 0) in erase step 308, and is then subjected, in a second resetstep designated 304′, to six reset pulses, alternately white and black,so that at the end of this reset step 304′, the pixel has returned to ablack state. Finally, in a second writing step designated 306′, thepixel is written with the appropriate gray level for a second image,assumed to be level 2.

[0170] Numerous variations of the drive scheme shown in FIG. 9 are ofcourse possible. One useful variation is shown in FIG. 10. The steps304, 306 and 308 shown in FIG. 10 are identical to those shown in FIG.9. However, in step 304′, five reset pulses are used (obviously adifferent odd number of pulses could also be used), so that at the endof step 304′, the pixel is in a white state (level 3), and in the secondwriting step 306′, writing of the pixel is effected from this whitestate rather than the black state as in FIG. 9. Successive images arethen written alternately from black and white states of the pixel.

[0171] In a further variation of the drive schemes shown in FIGS. 9 and10, erase step 308 is effected to as to drive the pixel white (level 3)rather than black. An even number of reset pulses are then applied tothat the pixel ends the reset step in a white state, and the secondimage is written from this white state. As with the drive scheme shownin FIG. 10, in this scheme successive images are written alternatelyfrom black and white states of the pixel.

[0172] It will be appreciated that in all the foregoing schemes, thenumber and duration of the reset pulses can be varied depending upon thecharacteristics of the electro-optic medium used. Similarly, voltagemodulation rather than pulse width modulation may be used to vary theimpulse applied to the pixel.

[0173] The black and white flashes which appear on the display duringthe reset steps of the drive schemes described above are of coursevisible to the user and may be objectionable to many users. To lessenthe visual effect of such reset steps, it is convenient to divide thepixels of the display into two (or more) groups and to apply differenttypes of reset pulses to the different groups. More specifically, if itnecessary to use reset pulses which drive any given pixel alternatelyblack and white, it is convenient to divide the pixels into at least twogroups and to arrange the drive scheme so that one group of pixels aredriven white at the same time that another group are driven black.Provided the spatial distribution of the two groups is chosen carefullyand the pixels are sufficiently small, the user will experience thereset step as an interval of gray on the display (with perhaps someslight flicker), and such a gray interval is typically lessobjectionable than a series of black and white flashes.

[0174] For example, in one form of such a “two group reset” step, thepixel in odd-numbered columns may be assigned to one “odd” group and thepixels in the even-numbered columns to the second “even” group. The oddpixels could then make use of the drive scheme shown in FIG. 9, whilethe even pixels could make use of a variant of this drive scheme inwhich, during the erase step, the pixels are driven to a white rather ablack state. Both groups of pixels would then be subjected to an evennumber of reset pulses during reset step 304′, so that the reset pulsesfor the two groups are essentially 180° out of phase, and the displayappears gray throughout this reset step. Finally, during the writing ofthe second image at step 306′, the odd pixels are driven from black totheir final state, while the even pixels are driven from white to theirfinal state. In order to ensure that every pixel is reset in the samemanner over the long term (and thus that the manner of resetting doesnot introduce any artifacts on to the display), it is advantageous forthe controller to switch the drive schemes between successive images, sothat as a series of new images are written to the display, each pixel iswritten to its final state alternately from black and white states.

[0175] Obviously, a similar scheme can be used in which the pixels inodd-numbered rows form the first group and the pixels in even-numberedrows the second group. In a further similar drive scheme, the firstgroup comprises pixels in odd-numbered columns and odd-numbered rows,and even-numbered columns and even-numbered rows, while the second groupcomprises in odd-numbered columns and even-numbered rows, andeven-numbered columns and odd-numbered rows, so that the two groups aredisposed in a checkerboard fashion.

[0176] Instead of or in addition to dividing the pixels into two groupsand arranging for the reset pulses in one group to be 180° out of phasewith those of the other group, the pixels may be divided into groupswhich use different reset steps differing in number and frequency ofpulses. For example, one group could use the six pulse reset sequenceshown in FIG. 9, while the second could use a similar sequence havingtwelve pulses of twice the frequency. In a more elaborate scheme, thepixels could be divided into four groups, with the first and secondgroups using the six pulse scheme but 180° out of phase with each other,while the third and fourth groups use the twelve pulse scheme but 180°out of phase with each other.

[0177] Another scheme for reducing the objectionable effects of resetsteps will now be described with reference to FIGS. 11A and 11B. In thisscheme, the pixels are again divided into two groups, with the first(even) group following the drive scheme shown in FIG. 11A and the second(odd) group following the drive scheme shown in FIG. 11B. Also in thisscheme, all the gray levels intermediate black and white are dividedinto a first group of contiguous dark gray levels adjacent the blacklevel, and a second group of contiguous light gray levels adjacent thewhite level, this division being the same for both groups of pixels.Desirably but not essentially, there are the same number of gray levelsin these two groups; if there are an odd number of gray levels, thecentral level may be arbitrarily assigned to either group. For ease ofillustration, FIGS. 11A and 11B show this drive scheme applied to aneight-level gray scale display, the levels being designated 0 (black) to7 (white); gray levels 1, 2 and 3 are dark gray levels and gray levels4, 5 and 6 are light gray levels.

[0178] In the drive scheme of FIGS. 11A and 11B, gray to graytransitions are handled according to the following rules:

[0179] (a) in the first, even group of pixels, in a transition to a darkgray level, the last pulse applied is always a white-going pulse (i.e.,a pulse having a polarity which tends to drive the pixel from its blackstate to its white state), whereas in a transition to a light graylevel, the last pulse applied is always a black-going pulse;

[0180] (b) in the second, odd group of pixels, in a transition to a darkgray level, the last pulse applied is always a black-going pulse,whereas in a transition to a light gray level, the last pulse applied isalways a white-going pulse;

[0181] (c) in all cases, a black-going pulse may only succeed awhite-going pulse after a white state has been attained, and awhite-going pulse may only succeed a black-going pulse after a blackstate has been attained; and

[0182] (d) even pixels may not be driven from a dark gray level to blackby a single black-going pulse nor odd pixels from a light gray level towhite using a single white-going pulse.

[0183] (Obviously, in both cases, a white state can only be achievedusing a final white-going pulse and a black state can only be achievedusing a final black-going pulse.)

[0184] The application of these rules allows each gray to graytransition to be effected using a maximum of three successive pulses.For example, FIG. 11A shows an even pixel undergoing a transition fromblack (level 0) to gray level 1. This is achieved with a singlewhite-going pulse (shown of course with a positive gradient in FIG. 11A)designated 1102. Next, the pixel is driven to gray level 3. Since graylevel 3 is a dark gray level, according to rule (a) it must be reachedby a white-going pulse, and the level 1/level 3 transition can thus behandled by a single white-going pulse 1104, which has an impulsedifferent from that of pulse 1102.

[0185] The pixel is now driven to gray level 6. Since this is a lightgray level, it must, by rule (a) be reached by a black-going pulse.Accordingly, application of rules (a) and (c) requires that this level3/level 6 transition be effected by a two-pulse sequence, namely a firstwhite-going pulse 1106, which drives the pixel white (level 7), followedby a second black-going pulse 1108, which drives the pixel from level 7to the desired level 6.

[0186] The pixel is next driven to gray level 4. Since this is a lightgray level, by an argument exactly similar to that employed for thelevel 1/level 3 transition discussed earlier, the level 6/level 4transition is effected by a single black-going pulse 1110. The nexttransition is to level 3. Since this is a dark gray level, by anargument exactly similar to that employed for the level 3/level 6transition discussed earlier, the level 4/level 3 transition is handledby a two-pulse sequence, namely a first black-going pulse 1112, whichdrives the pixel black (level 0), followed by a second white-going pulse1114, which drives the pixels from level 0 to the desired level 3.

[0187] The final transition shown in FIG. 11A is from level 3 tolevel 1. Since level 1 is a dark gray level, it must, according to rule(a) be approached by a white-going pulse. Accordingly, applying rules(a) and (c), the level 3/level 1 transition must be handled by athree-pulse sequence comprising a first white-going pulse 1116, whichdrives the pixel white (level 7), a second black-going pulse 1118, whichdrives the pixel black (level 0), and a third white-going pulse 1120,which drives the pixel from black to the desired level 1 state.

[0188]FIG. 11B shows an odd pixel effecting the same 0-1-3-6-4-3-1sequence gray states as the even pixel in FIG. 11A. It will be seen,however, that the pulses sequences employed are very different. Rule (b)requires that level 1, a dark gray level, be approached by a black-goingpulse. Hence, the 0-1 transition is effected by a first white-goingpulse 1122, which drives the pixel white (level 7), followed by ablack-going pulse 1124, which drives the pixel from level 7 to thedesired level 1. The 1-3 transition requires a three-pulse sequence, afirst black-going pulse 1126, which drives the pixel black (level 0), asecond white-going pulse 1128, which drives the pixel white (level 7),and a third black-going pulse 1130, which drives the pixel from level 7to the desired level 3. The next transition is to level 6 is a lightgray level, which according to rule (b) is approached by a white-goingpulse, the level 3/level 6 transition is effected by a two-pulsesequence comprising a black-going pulse 1132, which drives the pixelblack (level 0), and a white-going pulse 134, which drives the pixel tothe desired level 6. The level 6/level 4 transition is effected by athree-pulse sequence, namely a white-going pulse 1136, which drives thepixel white (level 7), a black-going pulse 1138, which drives the pixelblack (level 0) and a white-going pulse 1140, which drives the pixel tothe desired level 4. The level 4/level transition 3 transition iseffected by a two-pulse sequence comprising a white-going pulse 1142,which drives the pixel white (level 7), followed by a black-going pulse1144, which drives the pixel to the desired level 3. Finally, the level3/level 1 transition is effected by a single black-going pulse 1146.

[0189] It will be seen from FIGS. 11A and 11B that this drive schemeensures that each pixel follows a “sawtooth” pattern in which the pixeltravels from black to white without change of direction (althoughobviously the pixel may rest at any intermediate gray level for a shortor long period), and thereafter travels from white to black withoutchange of direction. Thus, rules (c) and (d) above may be replaced by asingle rule (e) as follows:

[0190] (e) once a pixel has been driven from one extreme optical state(i.e., white or black) towards the opposed extreme optical state by apulse of one polarity, the pixel may not receive a pulse of the opposedpolarity until it has reached the aforesaid opposed extreme opticalstate.

[0191] Thus, this drive scheme ensures that a pixel can only undergo, atmost, a number of transitions equal to (N−1)/2 transitions, where N isthe number of gray levels, before being driven to one extreme opticalstate; this prevents slight errors in individual transitions (caused,for example, by unavoidable minor fluctuations in voltages applied bydrivers) accumulating indefinitely to the point where serious distortionof a gray scale image is apparent to an observer. Furthermore, thisdrive scheme is designed so that even and odd pixels always approach agiven intermediate gray level from opposed directions, i.e., the finalpulse of the sequence is white-going in one case and black-going in theother. If a substantial area of the display, containing substantiallyequal numbers of even and odd pixels, is being written to a single graylevel, this “opposed directions” feature minimizes flashing of the area.

[0192] For reasons similar to those discussed above relating to otherdrive schemes which divide pixels into two discrete groups, whenimplementing the sawtooth drive scheme of FIGS. 11A and 11B, carefulattention should be paid to the arrangements of the pixels in the evenand odd groups. This arrangement will desirably ensure that anysubstantially contiguous area of the display will contain asubstantially equal number of odd and even pixels, and that the maximumsize of a contiguous block of pixels of the same group is sufficientlysmall not to be readily discernable by an average observer. As alreadydiscussed, arranging the two groups of pixels in a checkerboard patternmeets these requirements. Stochastic screening techniques may also beemployed to arrange the pixels of the two groups.

[0193] However, in this sawtooth drive scheme, use of a checkerboardpattern tends to increase the energy consumption of the display. In anygiven column of such a pattern, adjacent pixels will belong to oppositegroups, and in a contiguous area of substantial size in which all pixelsare undergoing the same gray level transition (a not uncommonsituation), the adjacent pixels will tend to require impulses ofopposite polarity at any given time. Applying impulses of oppositepolarity to consecutive pixels in any column requires discharging andrecharging the column (source) electrodes of the display as each newline is written. It is well known to those skilled in driving activematrix displays that discharging and recharging column electrodes is amajor factor in the energy consumption of a display. Hence, acheckerboard arrangement tends to increase the energy consumption of thedisplay.

[0194] A reasonable compromise between energy consumption and the desireto avoid large contiguous areas of pixels of the same group is to havepixels of each group assigned to rectangles, the pixels of which all liein the same column but extend for several pixels along that column. Withsuch an arrangement, when rewriting areas having the same gray level,discharging and recharging of the column electrodes will only benecessary when shifting from one rectangle to the next. Desirably, therectangles are 1×4 pixels, and are arranged so that rectangles inadjacent columns do not end on the same row, i.e., the rectangles inadjacent columns should have differing “phases”. The assignment ofrectangles in columns to phases may be effected either randomly or in acyclic manner.

[0195] One advantage of the sawtooth drive scheme shown in FIGS. 11A and11B is that any areas of the image which are monochrome are simplyupdated with a single pulse, either black to white or white to black, aspart of the overall updating of the display. The maximum time taken forrewriting such monochrome areas is only one-half of the maximum time forrewriting areas which require gray to gray transitions, and this featurecan be used to advantage for rapid updating of image features such ascharacters input by a user, drop-down menus etc. The controller cancheck whether an image update requires any gray to gray transitions; ifnot, the areas of the image which need rewriting can be rewritten usingthe rapid monochrome update mode. Thus, a user can have fast updating ofinput characters, drop-down menus and other user-interaction features ofthe display seamlessly superimposed upon a slower updating of a generalgrayscale image.

[0196] As discussed in the aforementioned copending applications Ser.Nos. 09/561,424 and 09/520,743, in many electro-optic media, especiallyparticle-based electrophoretic media, it is desirable that the drivescheme used to drive such media be direct current (DC) balanced, in thesense that, over an extended period, the algebraic sum of the currentspassed through a specific pixel should be zero or as close to zero aspossible, and the drive schemes of the present invention should bedesigned with this criterion in mind. More specifically, look-up tablesused in the present invention should be designed so that any sequence oftransitions beginning and ending in one extreme optical state (black orwhite) of a pixel should be DC balanced. From what has been said above,it might at first appear that such DC balancing may not be achievable,since the impulse, and thus the current through the pixel, required forany particular gray to gray transition is substantially constant.However, this is only true to a first approximation, and it has beenfound empirically that, at least in the case of particle-basedelectrophoretic media (and the same appears to be true of otherelectro-optic media), the effect of (say) applying five spaced 50 msecpulses to a pixel is not the same as applying one 250 msec pulse of thesame voltage. Accordingly, there is some flexibility in the currentwhich is passed through a pixel to achieve a given transition, and thisflexibility can be used to assist in achieving DC balance. For example,the look-up table used in the present invention can store multipleimpulses for a given transition, together with a value for the totalcurrent provided by each of these impulses, and the controller canmaintain, for each pixel, a register arranged to store the algebraic sumof the impulses applied to the pixel since some prior time (for example,since the pixel was last in a black state). When a specific pixel is tobe driven from a white or gray state to a black state, the controllercan examine the register associated with that pixel, determine thecurrent required to DC balance the overall sequence of transitions fromthe previous black state to the forthcoming black state, and choose theone of the multiple stored impulses for the white/gray to blacktransition needed which will either accurately reduce the associatedregister to zero, or at least to as small a remainder as possible (inwhich case the associated register will retain the value of thisremainder and add it to the currents applied during later transitions).It will be apparent that repeated applications of this process canachieve accurate long term DC balancing of each pixel.

[0197] It should be noted that the sawtooth drive scheme shown in FIGS.11A and 11B is well adapted for use of such a DC balancing technique, inthat this drive scheme ensures that only a limited number of transitionscan elapse between successive passes of any given pixel through theblack state, and indeed that on average a pixel will pass through theblack state on one-half of its transitions.

[0198] The objectionable effects of reset steps may be further reducedby using local rather than global updating, i.e., by rewriting onlythose portions of the display which change between successive images,the portions to be rewritten being chosen on either a “local area” or apixel-by-pixel basis. For example, it is not uncommon to find a seriesof images in which relatively small objects move against a larger staticbackground, as for example in diagrams illustrating movement of parts inmechanical devices or diagrams used in accident reconstruction. To uselocal updating, the controller needs to compare the final image with theinitial image and determine which area(s) differ between the two imagesand thus need to be rewritten. The controller may identify one or morelocal areas, typically rectangular areas having sides aligned with thepixel grid, which contain pixels which need to be updated, or may simplyidentify individual pixels which need to be updated. Any of the drivescheme already described may then be applied to update only the localareas or individual pixels thus identified as needing rewriting. Such alocal updating scheme can substantially reduce the energy consumption ofa display.

[0199] The aforementioned drive schemes may be varied in numerous waysdepending upon the characteristics of the specific electro-optic displayused. For example, in some cases it may be possible to eliminate many ofthe reset steps in the drives schemes described above. For example, ifthe electro-optic medium used is bistable for long periods (i.e., thegray levels of written pixels change only very slowly with time) and theimpulse needed for a specific transition does not vary greatly with theperiod for which the pixel has been in its initial gray state, thelook-up table may be arranged to effect gray state to gray statetransitions directly without any intervening, return to a black or whitestate, resetting of the display being effected only when, after asubstantial period has elapsed, the gradual “drift” to pixels from theirnominal gray levels could have caused noticeable errors in the imagepresented. Thus, for example, if a user was using a display of thepresent invention as an electronic book reader, it might be possible todisplay numerous screens of information before resetting of the displaywere necessary; empirically, it has been found that with appropriatewaveforms and drivers, as many as 1000 screens of information can bedisplayed before resetting is necessary, so that in practice resettingwould not be necessary during a typical reading session of an electronicbook reader.

[0200] It will readily be apparent to those skilled in displaytechnology that a single apparatus of the present invention couldusefully be provided with a plurality of different drive schemes for useunder differing conditions. For example, since in the drive schemesshown in FIGS. 9 and 10, the reset pulses consume a substantial fractionof the total energy consumption of the display, a controller might beprovided with a first drive scheme which resets the display at frequentintervals, thus minimizing gray scale errors, and a second scheme whichresets the display only at longer intervals, thus tolerating greatergray scale errors but reduce energy consumption. Switching between thetwo schemes can be effected either manually or dependent upon externalparameters; for example, if the display were being used in a laptopcomputer, the first drive scheme could be used when the computer isrunning on mains electricity, while the second could be used while thecomputer was running on internal battery power.

[0201] From the foregoing description, it will be seen that the presentinvention provides a driver for controlling the operation ofelectro-optic displays, which are well adapted to the characteristics ofbistable particle-based electrophoretic displays and similar displays.

[0202] From the foregoing description, it will also be seen that thepresent invention provides a method and controller for controlling theoperation of electro-optic displays which allow accurate control of grayscale without requiring inconvenient flashing of the whole display toone of its extreme states at frequent intervals. The present inventionalso allows for accurate control of the display despite changes in thetemperature and operating time thereof, while lowering the powerconsumption of the display. These advantages can be achievedinexpensively, since the controller can be constructed from commerciallyavailable components.

[0203] In the remnant voltage method of the present invention,measurement of the remnant voltage is desirably effected by a highimpedance voltage measurement device, for example a metal oxidesemiconductor (MOS) comparator. When the display is one having smallpixels, for example a 100 dots per inch (DPI) matrix display, in whicheach pixel has an area of 10⁻⁴ square inch or about 6×10⁻² mm², thecomparator needs to have an ultralow input current, as the resistance ofsuch a single pixel is of the order of 10¹² ohm. However, suitablecomparators are readily available commercially; for example, the TexasInstruments INA111 chip is suitable, as it has an input current on onlyabout 20 pA. (Technically, this integrated circuit is an instrumentationamplifier, but if its output is routed into a Schmitt trigger, it actsas a comparator.) For displays having large single pixels, such as largedirect-drive displays (defined below) used in signage, where theindividual pixels may have areas of several square centimeters, therequirements for the comparator are much less stringent, and almost anycommercial FET input comparator may be used, for example the LF311comparator from National Semiconductor Corporation.

[0204] It will readily be apparent to those skilled in the art ofelectronic displays that, for cost and other reasons, mass-producedelectronic displays will normally have drivers in the form ofapplication specific integrated circuits (ASIC's), and in this type ofdisplay the comparator would typically be provided as part of the ASIC.Although this approach would require provision of feedback circuitrywithin the ASIC, it would have the advantage of making the power supplyand oscillator sections of the ASIC simpler and smaller in area. Iftri-level general image flow drive is required, this approach would alsomake the driver section of the ASIC simpler and smaller in area. Thus,this approach would typically reduce the cost of the ASIC.

[0205] Conveniently, a driver which can apply a driving voltage,electronically short or float the pixel, is used to apply the drivingpulses. When using such a driver, on each addressing cycle where DCbalance correction is to be effected, the pixel is addressed,electronically shorted, then floated. (The term “addressing cycle” isused herein in its conventional meaning in the art of electro-opticdisplays to refer to the total cycle needed to change from a first to asecond image on the display. As noted above, because of the relativelylow switching speeds of electrophoretic displays, which are typically ofthe order of tens to hundreds of milliseconds, a single addressing cyclemay comprise a plurality of scans of the entire display.) After a shortdelay time, the comparator is used to measure the remnant voltage acrossthe pixel, and to determine whether it is positive or negative in sign.If the remnant voltage is positive, the controller may slightly extendthe duration of (or slightly increase the voltage of) negative-goingaddressing pulses on the next addressing cycle. If, however, the remnantvoltage is negative, the controller may slightly extend the duration of(or slightly increase the voltage of) positive-going voltage pulses onthe next addressing cycle.

[0206] Thus, the remnant voltage method of the invention places theelectro-optic medium into a bang-bang feedback loop, adjusting thelength of the addressing pulses to drive the remnant voltage towardzero. When the remnant voltage is near zero, the medium exhibits idealperformance and improved lifetime. In particular, use of the presentinvention may allow improved control of gray scale. As noted earlier, ithas been observed that the gray scale level obtained in electro-opticdisplays is a function not only of the starting gray scale level and theimpulse applied, but also of the previous states of the display. It isbelieved (although this invention is in no way limited by this belief)that one of the reasons for this “history” effect on gray scale level isthat the remnant voltage affects the electric field experienced by theelectro-optic medium; the actual electric field influencing the behaviorof the medium is the sum of the voltage actually applied via theelectrodes and the remnant voltage. Thus, controlling the remnantvoltage in accordance with the present invention ensures that theelectric field experienced by the electro-optic medium accuratelycorresponds to that applied via the electrodes, thus permitting improvedcontrol of gray scale.

[0207] The remnant voltage method of the present invention is especiallyuseful in displays of the so-called “direct drive” type, which aredivided into a series of pixels each of which is provided with aseparate electrode, the display further comprising switching meansarranged to control independently the voltage applied to each separateelectrode. Such direct drive displays are useful for the display of textor other limited character sets, for example numerical digits, and aredescribed in, interalia, the aforementioned International ApplicationPublication No. 00/05704. However, the remnant voltage method of thepresent invention can also be used with other types of displays, forexample active matrix displays which use an array of transistors, atleast one of which is associated with each pixel of the display.Activating the gate line of a thin film transistor (TFT) used in such anactive matrix display connects the pixel electrode to the sourceelectrode. The remnant voltage is small compared to the gate voltage(the absolute value of the remnant voltage typically does not exceedabout 0.5 V), so the gate drive voltage will still turn the TFT on. Thesource line can then be electronically floated and connected to a MOScomparator, thus allowing reading the remnant voltage of each individualpixel of the active matrix display.

[0208] It should be noted that, although the remnant voltage on a pixelof an electrophoretic display does closely correlate with the extent towhich the current flow through that pixel has been DC-balanced, zeroremnant voltage does not necessarily imply perfect DC-balance. However,from the practical point of view, this makes little difference, since itappears to be the remnant voltage itself rather than the DC-balancehistory which is responsible for the adverse effects noted herein.

[0209] It will readily be apparent to those skilled in the display artthat, since the purpose of the remnant voltage method of the presentinvention is to reduce remnant voltage and DC imbalance, this methodneed not be applied on every addressing cycle of a display, provided itis applied with sufficient frequency to prevent a long-term build-up ofDC imbalance at a particular pixel. For example, if the display is onewhich requires use of a “refresh” or “blanking” pulse at intervals, suchthat during the refresh or blanking pulse all of the pixels are drivento the same display state, normally one of the extreme display states(or, more commonly, all of the pixels are first driven to one extremedisplay state, and then to the other extreme display state), the methodof the present invention might be practiced only during the refresh orblanking pulses.

[0210] Although the remnant voltage method of the invention hasprimarily been described in its application to encapsulatedelectrophoretic displays, this method may be also be used withunencapsulated electrophoretic displays, and with other types ofdisplay, for example electrochromic displays, which display a remnantvoltage.

[0211] From the foregoing description, it will be seen that the remnantvoltage method of the present invention provides a method for drivingelectrophoretic and other electro-optic displays which reduces the costof the equipment needed to ensure DC balancing of the pixels of thedisplay, while providing increasing display lifetime, operating windowand long-term display optical performance.

[0212] Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the spirit and skill of the invention. Accordingly, the foregoingdescription is to be construed in an illustrative and not in alimitative sense.

1. A method of driving a bistable electro-optic display having aplurality of pixels, each of which is capable of displaying at leastthree gray levels, the method comprising: storing a look-up tablecontaining data representing the impulses necessary to convert aninitial gray level to a final gray level; storing data representing atleast an initial state of each pixel of the display; receiving an inputsignal representing a desired final state of at least one pixel of thedisplay; and generating an output signal representing the impulsenecessary to convert the initial state of said one pixel to the desiredfinal state thereof, as determined from said look-up table.
 2. A methodaccording to claim 1 further comprising storing data representing atleast prior state of each pixel prior to said initial state thereof, andwherein said output signal is generated dependent upon both said atleast one prior state and said initial state of said one pixel.
 3. Amethod according to claim 2 wherein data is stored representing at leasttwo prior states of each pixel and said output signal is generateddependent upon said at least two prior state and said initial state ofsaid one pixel.
 4. A method according to claim 1 further comprisingreceiving a temperature signal representing the temperature of at leastone pixel of the display and generating said output signal dependentupon said temperature signal.
 5. A method according to claim 4 whereinsaid look-up table stores multiple values for each transition from aninitial gray level to a final gray level, said multiple valuesrepresenting the values required for a specific transition at a specifictemperature.
 6. A method according to claim 5 further comprisinginterpolating between adjacent values for a transition when thetemperature signal indicates a temperature intermediate the temperaturesto which said adjacent values relate.
 7. A method according to claim 4wherein said look-up table stores functions of temperature, and whereinsaid output signal is generated by calculating the value of the relevantfunction at the temperature indicated by said temperature signal.
 8. Amethod according to claim 1 further comprising generating a lifetimesignal representing the operating time of said pixel and generating saidoutput signal dependent upon said lifetime signal.
 9. A method accordingto claim 1 further comprising generating a residence time signalrepresenting the time since said pixel last underwent a transition andgenerating said output signal dependent upon said residence time signal.10. A method according to claim 1 wherein said output signal representsthe period of time for which a substantially constant drive voltage isto be applied to said pixel.
 11. A method according to claim 10 whereinsaid pixel is driven in a scan comprising a plurality of sub-scanperiods and said output signal represents determines during which ofsaid sub-scan periods a drive voltage is to be applied to said pixel.12. A method according to claim 1 wherein said output signal comprisesat least one polarity bit representing the polarity of the impulsenecessary to convert the initial state of said one pixel to the desiredfinal state thereof.
 13. A method of driving an electro-optic displayhaving a plurality of pixels, each of which is capable of displaying atleast three gray levels, the method comprising: storing a look-up tablecontaining data representing the impulses necessary to convert aninitial gray level to a final gray level; storing data representing atleast an initial state of each pixel of the display; receiving an inputsignal representing a desired final state of at least one pixel of thedisplay; and generating an output signal representing the impulsenecessary to convert the initial state of said one pixel to the desiredfinal state thereof, as determined from said look-up table, said outputsignal representing the period of time for which a substantiallyconstant drive voltage is to be applied to said pixel.
 14. A devicecontroller for controlling a bistable electro-optic display having aplurality of pixels, each of which is capable of displaying at leastthree gray levels, said controller comprising: storage means arranged tostore both a look-up table containing data representing the impulsesnecessary to convert an initial gray level to a final gray level, anddata representing at least an initial state of each pixel of thedisplay; input means for receiving an input signal representing adesired final state of at least one pixel of the display; calculationmeans for determining, from the input signal, the stored datarepresenting the initial state of said pixel, and the look-up table, theimpulse required to change the initial state of said one pixel to thedesired final state; and output means for generating an output signalrepresentative of said impulse.
 15. A controller according to claim 14wherein said storage means is also arranged to store data representing aleast one prior state of each pixel prior to said initial state thereof,and said calculation means is arranged to determine said impulsedependent upon said input signal, said initial state of said pixel andsaid prior state of said pixel.
 16. A controller according to claim 15wherein said storage means is arranged to store data representing atleast two prior states of each pixel, and said calculation means isarranged to determine said impulse dependent upon said input signal,said initial state of said pixel and said at least two prior states ofsaid pixel.
 17. A controller according to claim 14 wherein said inputmeans is arranged to receive a temperature signal representing thetemperature of at least one pixel of the display, and said calculationmeans is arranged to determine said impulse dependent upon said inputsignal, said initial state of said pixel and said temperature signal.18. A controller according to claim 17 wherein said storage means isarranged to store multiple values for the impulses necessary to convertan initial gray level to a final gray level, said multiple valuesrepresenting the values required for a specific transition at a specifictemperature.
 19. A controller according to claim 18 wherein saidcalculation means is arranged to interpolate between adjacent ones ofsaid stored multiple values when said temperature signal indicates atemperature intermediate the temperatures to which said adjacent storedvalues relate.
 20. A controller according to claim 17 wherein saidstorage means is arranged to store functions of temperature, and saidcalculation means is arranged to determine said impulse by calculatingthe value of the relevant function at the temperature represented bysaid temperature signal.
 21. A controller according to claim 14 furthercomprising lifetime signal generation means arranged to generate alifetime signal representing the operating time of said pixel, saidcalculation means determining said impulse from said input signal, saidstored data representing the initial state of said pixel and saidlifetime signal.
 22. A controller according to claim 14 furthercomprising residence time signal generation means for determining theresidence time since said pixel last underwent a transition and forgenerating a residence time signal representing said residence time,said calculation means determining said impulse from said input signal,said stored data representing the initial state of said pixel and saidresidence time signal.
 23. A controller according to claim 14 whereinsaid output means generates a signal representing the time for which asubstantially constant drive voltage is to be applied to said pixel. 24.A controller according to claim 14 wherein said output signal comprisesat least one polarity bit representing the polarity of said impulse. 25.A method of driving a bistable electro-optic display having a pluralityof pixels, each of which is capable of displaying at least three graylevels, the method comprising: storing a look-up table containing datarepresenting the impulses necessary to convert an initial gray level toa final gray level; storing data representing at least an initial stateof each pixel of the display; receiving an input signal representing adesired final state of at least one pixel of the display; and generatingan output signal representing the impulse necessary to convert theinitial state of said one pixel to the desired final state thereof, asdetermined from said look-up table, the output signal representing theperiod of time for which a substantially constant drive voltage is to beapplied to said pixel.
 26. A device controller for controlling anelectro-optic display having a plurality of pixels, each of which iscapable of displaying at least three gray levels, said controllercomprising: storage means arranged to store both a look-up tablecontaining data representing the impulses necessary to convert aninitial gray level to a final gray level, and data representing at leastan initial state of each pixel of the display; input means for receivingan input signal representing a desired final state of at least one pixelof the display; calculation means for determining, from the inputsignal, the stored data representing the initial state of said pixel,and the look-up table, the impulse required to change the initial stateof said one pixel to the desired final state; and output means forgenerating an output signal representative of said impulse, said outputsignal representing the period of time for which a substantiallyconstant drive voltage is to be applied to said pixel.
 27. A devicecontroller comprising: storage means arranged to store both a look-uptable containing data representing the impulses necessary to convert aninitial gray level to a final gray level, and data representing at leastan initial state of each pixel of the display; input means for receivingan input signal representing a desired final state of at least one pixelof the display; calculation means for determining, from the inputsignal, the stored data representing the initial state of said pixel,and the look-up table, the impulse required to change the initial stateof said one pixel to the desired final state; and output means forgenerating an output signal representative of said impulse, the outputsignal representing a plurality of pulses varying in at least one ofvoltage and duration, the output signal representing a zero voltageafter the expiration of a predetermined period of time.
 28. A drivercircuit comprising: output lines arranged to be connected to driveelectrodes of an electro-optic display; first input means for receivinga plurality of (n+1) bit numbers representing the voltage and polarityof signals to be placed on the drive electrodes; and second input meansfor receiving a clock signal, the driver circuit being arranged suchthat, upon receipt of the clock signal, the driver circuit displays theselected voltages on its output lines.
 29. A driver circuit comprising:output lines arranged to be connected to drive electrodes of anelectro-optic display; first input means for receiving a plurality of2-bit numbers representing the voltage and polarity of signals to beplaced on the drive electrodes; and second input means for receiving aclock signal, the driver circuit being arranged such that, upon receiptof the clock signal, the driver circuit displays voltages selected fromR+V, R and R−V on its output lines, where R is a reference voltage and Vis the maximum difference from the reference voltage which the drivercircuit can assert.
 30. A method for driving an electro-optic displayhaving a remnant voltage, the method comprising: (a) applying a firstdriving pulse to a pixel of the display; (b) measuring the remnantvoltage of the pixel after the first driving pulse; and (c) applying asecond driving pulse to the pixel following the measurement of theremnant voltage, the magnitude of the second driving pulse beingcontrolled dependent upon the measured remnant voltage to reduce theremnant voltage of the pixel.